I personally believe the game's difficulty is balanced (and even too easy at times), but i can understand that some people can struggle.
The game DOESN'T require farming echoes (although you can, for example the forest level or imaginay prisons level have some nice payouts).
The game has a lot of ways to be played, there isn't only one approach. However i will be focusing on strategies that i personally used the most.
Most fights will look the same(if you choose to play this way). Stun, bleeding / cleave (or even both) and deal a lot of damage - quickly.

Here is a skilltree if you want how to get a certain skill or how much it costs(right click on it and open in new tab for better visibility):
PS. The skilltree graph is not mine, author preferred staying anon.

Here is base(no skills involved) weapons info (Needed echoes, Focus, and their overall stats):


PS. Sorry if the guide feels a bit bad, there is a lot of text and no videos. I wanted to publish it remotely fast, so people aren't struggling for no reason - the game will also be a bit easier in the future and will also include difficulty settings! Try to restrain from leaving a negative review simply because of the combat system.

The game has optional weapons that make things easier. They are however, not needed to complete the game.
The weapons are as follows: Revolver, Sledgehammer, Chainsaw.
They are easy to find and they drop from optional bosses.
Location of these weapons / bosses :
Revolver can be found after repeating The Hotel and going up the elevator.
tip: At the start of The Hotel you are faced against 3 shadows - you don't need to fight them. You can just escape the fight (about twice from my experience) and they won't be able to catch you in time.
After defeating the boss you will receive the Revolver.
Sledgehammer can be found by killing the optional boss in the Doll House.
You can find her by putting the coin into the capsule toy dispenser (next to BERT!), examining a toy capsule, and using the key on the door in the same room. In the following room you will be able to start the boss fight after interacting with the mirror.
Chainsaw can be found in the Cave.
After locating the boss, you need to cut the locks with the bolt cutters (2 interactions).

The game allows you to reset your skills, once per day, by interacting with the fridge.
USE that to your advantage since it makes everything way easier. There is no shame in resetting your skills.

To make things easier, you should have skills that work against whoever you're going against. You won't use 25 backpack space for a boss fight, and you won't need a cleave upgrade for a single target.

Tips:
A tip not really for the bosses, but the "escape" button in fights can be extremely important if you aren't prepared. You don't need to use it at all, but you can skip some fights that could give you an issue.
Try to not overstack the bleeding. If you apply 15 bleeding on an enemy, it won't have the time to kill them.
Do not block / heal if it's not needed. Prioritize the damage, it applies to all types of battles.
Addition to the tip above: healing in almost every situation is better than blocking. If you need to use any, use the heal.
Use haste - based weapons when the enemy has "0" on the attack, so you won't actually speed them up.
Try to always use a slow type of attack, especially against single targets.
Try planning a fight in a way you won't need to craft anything during it.
It's always better to stun the target while they have bleeding on them. If your weapons deal 2 bleeding, you should use it before the stun, so that additional bleeding gets used during the stun.
Don't overspend on healing / blocking. In most cases you won't need them, and a lot of the levels will give it to you for free.
As Mouxine in the comments mentioned, be careful with combining Double Vision and Sharp Mind. It doesn't do anything for 2 / 3 priced items (you won't get the benefit from Double Vision) it can still save you focus with each weapon costing 1 or above 3. You can spend the echoes elsewhere if you feel the benefit of combining them won't give you much (for example - on coffee addict)).
If you want to choose only one skill (not combining them together) Sharp Mind is definitely better!
The best way to spend your day is easily the TV. It will give you one of the researched items. It's better than every other option, especially in my strategies.

Single enemies are the easiest type of fights in this game.
The strategy is simple. You want to stun them as much as possible (wrench, camera) while also inflicting bleeding and raw damage.
It makes these fights pretty much trivial.

Good weapons for solo targets:
Pickaxe (2 raw damage, 1 bleeding) for 3 focus.
Nailed Plank (1 raw damage, 1 bleeding) for 2 focus.
Optional weapon: Revolver (5 raw damage) for 5 focus.
Spider's Fang ( 7 bleeding, 1 haste ) for 2 focus.
Wrench (1 raw damage, 1 slow) for 3 focus.
Camera (slow 2, slow others 2) for 4 focus. (I prefer the wrench, but this isn't a bad item for this one)
Human Bone (1 raw damage) for 1 focus.
Honorable mention:
The Coal for 2 focus.
It increases your luck (each charge increases it by X (unknown amount)) allowing to roll additional skills without having them for example: Lucky (that makes your attack not lose a charge)
Keep in mind it's a bit of RNG. You might have 0 value from it quite frequently, but if you have a spare focus (and you know you won't run out of damage) you might give it a try.

Your goal is to ALWAYS have some bleeding on your enemy. This matters a lot, because each time you're slowing them down, the bleeding deals them damage.
Spider's fang is essentially 7 damage for only 2 focus if done right.

Good skills for solo targets:
Precise Hit(requires integrity lvl 3 -> desperate punch):
Increases your damage by 1 per level 2000 for the first level and 5000 for the second level.
You can deal an insane amount of damage before your opponent even gets a chance to do anything, and makes your focus / damage very good.

Stunning Strike (requires integrity lvl 3 -> desperate punch) :
Increases your stun duration by 1, costs 4000.
Another insanely good skill, that allows you to attack more.

Swap Timelines:
Allows you to repeat the battles, costs 250. Technically not needed, but helps you out if you fail.

Sharp Mind (requires focus 4):
Allows you to craft items for 1 less focus (can't become 0), costs 2500.
Great skill as well, allowing you to craft more items before the boss fight.

Double Vision (requires focus 5):
Allows you to always craft 2 charges of an item, at the cost of it being 50% more expensive, costs 3000.
It is definitely worth it, you will spend less focus overall.

Coffee Addict (requires focus 3):
Allows you to have more focus overall, price of each level: 500 -> 1000 -> 2000 -> 3000 6500 in total.

RNG skills (Lucky, Dodge, Critical Hit):
They might be useful, but since they are RNG i don't recommend them (since all of the boss fights can be completed without them, making your run more RNG for no reason is usually not worth it).
If you have a lot of echoes you might give them a chance if you want.

Depending on how many echoes you have, you will unlock less / more skills. It's impossible to predict your amount of echoes so you need to improvise a little bit. Pick skills that will actually help you.

So your strategy would go something like this:
Use the Spider's Fang / other sources of poison very early on (info in tips section)
Then keep applying the slow with the Wrench and keep dealing raw damage. (For example, base Bone is 1 focus for 1 raw damage, but pickaxe is only 2 raw damage for 3 focus - and adding the bleed in this case won't help)
Keep the bleeding on at all times. These fights are very simple, you got this.

Sirens doesn't deal too much damage but they are healing. There are 3 of them.

You can choose 2 approaches:
Killing each siren before moving to the next one (kinda like a single target, while still stunning the rest with a camera for example).
Killing them all at the same time (your damage / focus will be better with cleave skills, but they will also heal more).
With the first approach you will play it out like a single target fight, but you need to be careful about other sirens (camera is great for it).
I prefer the second approach, but both are doable.

Good weapons for Sirens, second approach:
Metal Pipe (1 raw damage, 1 cleave) for 2 focus.
Cleaver (1 raw damage, 1 cleave, 1 bleed) for 3 focus.
Optional weapon: Chainsaw (3 raw damage, 1 cleave, 1 bleed, 1 bleed others) for 5 focus.
Optional weapon: Sledgehammer (2 raw damage, 1 cleave, 2 stun, 1 stun others)
Camera (slow 2, slow others 2) for 4 focus.
Rusty Axe (2 raw damage, 1 cleave, 2 bleed) for 4 focus.
Other usual weapons (like pickaxe, nailed plank etc) are good too, but won't be as effective.
Make sure to not run out of damage.
Honorable mention:
The Coal for 2 focus.
It increases your luck (each charge increases it by X (unknown amount)) allowing to roll additional skills without having them for example: Lucky (that makes your attack not lose a charge)
Keep in mind it's a bit of RNG. You might have 0 value from it quite frequently, but if you have a spare focus (and you know you won't run out of damage) you might give it a try.

Your goal is to keep them stunned, and keep dealing so much damage that they won't be able to heal / fight back quickly enough.

Good skills for Sirens, second approach:

Stunning Strike (requires integrity lvl 3 -> desperate punch) :
Increases your stun duration by 1, costs 4000.
Another insanely good skill, that allows you to attack more.

Wide Swing (requires integrity lvl 3 -> desperate punch -> Precise Hit 1) :
It allows you to deal more damage to everyone, costs 1000 for the first level and 3000 for the second one (4000 in total and 1 damage per level).

Swap Timelines:
Allows you to repeat the battles, costs 250. Technically not needed, but helps you out if you fail.

Sharp Mind (requires focus 4):
Allows you to craft items for 1 less focus (can't become 0), costs 2500.
Great skill as well, allowing you to craft more items before the boss fight.

Double Vision (requires focus 5):
Allows you to always craft 2 charges of an item, at the cost of it being 50% more expensive, costs 3000.
It is definitely worth it, you will spend less focus overall.

Coffee Addict (requires focus 3):
Allows you to have more focus overall, price of each level: 500 -> 1000 -> 2000 -> 3000 6500 in total (i would stop at lvl 2 or 3, you can go more if needed).

RNG skills (Lucky, Dodge, Critical Hit):
They might be useful, but since they are RNG i don't recommend them (since all of the boss fights can be completed without them, making your run more RNG for no reason is usually not worth it).
If you have a lot of echoes you might give them a chance if you want.

Depending on how many echoes you have, you will unlock less / more skills. It's impossible to predict your amount of echoes so you need to improvise a little bit. Pick skills that will actually help you.

So your strategy would go something like this:
You want to kill them ASAP and overcome their healing.
The camera (or optional weapons mentioned above) will give you a lot of stun to everyone allowing you to make a LOT of attacks before they even attempt anything. It shouldn't take you long to kill them.

These type of bosses spawn enemies across the fight (which means they are not healing, or dealing damage)
The basic strategy is that you want to kill the spawned creatures fast enough, to keep dealing damage to the main Boss.
Bleeding on the boss is very important, so he can slowly die in the background.

Good weapons for them:
Pickaxe (2 raw damage, 1 bleeding) for 3 focus.
Nailed Plank (1 raw damage, 1 bleeding) for 2 focus.
Optional weapon: Revolver (5 raw damage) for 5 focus.
Optional weapon: Chainsaw (3 raw damage, 1 cleave, 1 bleed, 1 bleed others) for 5 focus.
Optional weapon: Sledgehammer (2 raw damage, 1 cleave, 2 stun, 1 stun others)
Camera (slow 2, slow others 2) for 4 focus.
Spider's Fang ( 7 bleeding, 1 haste ) for 2 focus.
Wrench (1 raw damage, 1 slow) for 3 focus.
Camera (slow 2, slow others 2) for 4 focus. (I prefer the wrench, but this isn't a bad item for this one)
Human Bone (1 raw damage) for 1 focus.
Some cleave weapons could be nice too. Not needed though.
Honorable mention:
The Coal for 2 focus.
It increases your luck (each charge increases it by X (unknown amount)) allowing to roll additional skills without having them for example: Lucky (that makes your attack not lose a charge)
Keep in mind it's a bit of RNG. You might have 0 value from it quite frequently, but if you have a spare focus (and you know you won't run out of damage) you might give it a try.

Your goal is to ALWAYS have some bleeding on the boss.
Spider's fang is essentially 7 damage for only 2 focus if done right.

Good skills for them:
Precise Hit(requires integrity lvl 3 -> desperate punch):
Increases your damage by 1 per level 2000 for the first level and 5000 for the second level.
You can deal an insane amount of damage before your opponent even gets a chance to do anything, and makes your focus / damage very good.

Stunning Strike (requires integrity lvl 3 -> desperate punch) :
Increases your stun duration by 1, costs 4000.
Another insanely good skill, that allows you to attack more.

Swap Timelines:
Allows you to repeat the battles, costs 250. Technically not needed, but helps you out if you fail.

Sharp Mind (requires focus 4):
Allows you to craft items for 1 less focus (can't become 0), costs 2500.
Great skill as well, allowing you to craft more items before the boss fight.

Double Vision (requires focus 5):
Allows you to always craft 2 charges of an item, at the cost of it being 50% more expensive, costs 3000.
It is definitely worth it, you will spend less focus overall.

Coffee Addict (requires focus 3):
Allows you to have more focus overall, price of each level: 500 -> 1000 -> 2000 -> 3000 6500 in total.

Not needed, but might be useful (especially for the first 3 enemies during Puppet Master fight)
Wide Swing (requires integrity lvl 3 -> desperate punch -> Precise Hit 1) :
It allows you to deal more damage to everyone, costs 1000 for the first level and 3000 for the second one (4000 in total and 1 damage per level).

RNG skills (Lucky, Dodge, Critical Hit):
They might be useful, but since they are RNG i don't recommend them (since all of the boss fights can be completed without them, making your run more RNG for no reason is usually not worth it).
If you have a lot of echoes you might give them a chance if you want.

Depending on how many echoes you have, you will unlock less / more skills. It's impossible to predict your amount of echoes so you need to improvise a little bit. Pick skills that will actually help you.

So your strategy would go something like this:
Inflicting bleeding with the fang whenever the situation is good (info in the tips)
When they spawn the enemy you want to kill it asap. Cleave weapons can also help dealing more damage to the boss, but they will have gaps in the minion spawns so it's not needed.
Each time you will get to the boss you will stun them dealing them more damage and more bleeding.

There is also a separate strategy for the puppet master, but it can be quite difficult to pull off. After killing two of the first three enemies, you can leave one alive, and grind focus by waiting the turns (if you have the skill for it). You can grind quite a bit while staying alive, to be prepared for the boss better. Be careful though.

You will be more experienced further into the game with more echoes as well. A lot of the rules from previous bosses still apply, use them accordingly!
Good luck in your attempts, it shouldn't take too much time!
If it's needed (although shouldn't) you can farm echoes by repeating levels (for example the forest or imaginary prisons), so you can be stronger for these fights.
You can alternatively also wait for the upcoming changes as i said earlier. The game is great, don't let the combat system take the fun from it!