Danmaku Kagura sure has some hard achievements. They require quite a lot of commitment and skill to obtain. But, what if you don't have the skill and don't want to spend a few hundred hours practicing? Don't worry, this guide has some shortcuts that will make the process quite easier.
This guide covers the following achievements: "Not Too Shabby", "Full Mitama Card Collection" (and by extension, all mitama card collection ones), and "The Usual Suspects".

Alright, let's start with the achievement that is most skill-oriented.

Not Too Shabby: Complete Re: Unknown X on Hard or higher difficulty.

Re: Unknown X is a 26/27 level song with a lot of fast combos, it can be difficult to keep up. In fact, this is the last achievement that I got before perfection, so I know it can cause some trouble for people wanting to perfect the game but not having the skill and will to practice for a long time. Here are some tips to make this process much more doable:

TL,DR:
Story Mode Underworld 11, Hard, 4 key, comfortable tempo
One of the following Mitama Cards: "A Day Without Incident Is Empty", "I Wish I Could Die Under Spring Flowers", "A Pure Dream World". Level does not matter.
Sakuya as the heroine.
Large Hitbox and Gauge Rate Up boosters.
Don't worry about score at all

1. Keys and Tempo
Update 1.9.1 brought us the ability to select 4 or 6 keys for every difficulty! Use this to your advantage and select the 4 key mode for easier patterns. Also, set the tempo to one that you're comfortable with. I personally go with 342 (1.8x), but feel free to find your own fit. Just don't set it too high, or you may not see enough notes and fail to react in time.
Also, obviously, select Hard. Do you want to deal with an X difficulty song?

2. Use Story Mode (Underworld 11)
If you play this song in Free Mode, you're relying totally on your skills, which can prove to be difficult. However, have you considered using the boosts story mode has? You may be discouraged because of the timing reduction, but trust me, the advantages are worth it.
Story mode has two stages featuring this song: Underworld 11 and Underworld 12. Underworld 12 locks the tempo to 360, so I recommend against it unless you're comfortable with it. You'll get used to the vertical notes quickly.

3. Loadout
To complete a stage, you need your Groove Gauge to be high enough. The score does not matter, so we should focus on timing window and groove gauge fill rate.
The stage effect is a moderate decrease, but we can counter act this with a great increase. Use one of the following mitama cards for this effect: "A Day Without Incident Is Empty", "I Wish I Could Die Under Spring Flowers", "A Pure Dream World". I advise against using one with the risky timing increase, as you are likely to miss a few notes and they slow the groove gauge fill rate.
As for the Heroine, choose Sakuya for another slight timing boost with no downside. (I'm not entirely sure that this matters, but it's better to have it if it does)
Also, I think increasing the timing window matters more than increasing the groove gauge fill rate, as you need to hit notes in the first place to fill the gauge.

4. Booster Items
These are the most important reason for using story mode. For the price of 8000 dream drops (which should be very few), you can get the Large Hitbox and Gauge Rate Up booster items. Pick them up!

5. Play!
Now you should have the largest margin of error. The rest is getting used to the song and playing well enough to clear the stage. You don't need too much precision, just enough so you don't lose too much gauge towards the end, where there's a very long and fast combo without breaks in between. You may refer to my attempt using this exact loadout to see the groove gauge change. Also, don't worry about the score, it's completely irrelevant!

This is also a difficult one, requiring you to get all 346 mitama cards in game. Some are locked behind challenging missions on challenging stages with challenging gimmicks. However, there are a few shortcuts you can take. I'm not going to list the best way of obtaining each of them, as (1) there are too many and (2) different people may find different things challenging. I'll just give some general tips that you can follow.

1. Use the Best Mitama Card for the Situation
Different mitama cards are made for different situations. Identify the stage's missions and gimmicks, then pick a mitama card that helps you complete the mission or counteract the gimmick. You can always check the game's recommendation function!
The shop in Human Village has some cards that increase card drop rate in danmaku stages, they definitely will come in handy.

2. Don't Bang Your Head Against a Wall
If you're having a very hard time, you may want to step away from the mission at hand and do another one. After completing some other stages and getting more mitama cards, you may find one that helps you tackle the difficulty you encountered earlier.

3. Use DLC Stages
Although all mitama cards are available from the base game, some are locked behind really hard stages. But what if we don't have to do those stages at all? Introducing DLC1-4! These four DLCs were released alongside the base game and add stages to Story Mode. These stages often have mitama cards as rewards, allowing you to bypass a base game stage or mission. After all, you only have to get the card once, no matter the source.

4. Use Booster Items
Booster items don't disqualify you from completing a stage or a mission. Use them for a boost if you need it! You should be able to reliably get dream drops, so don't be afraid to use them.

This achievement requires unlocking 19 characters. Unlike mitama cards, characters only have one unlock method, which means a difficult mission can stop you from getting this achievement with no way around it. Because of this, I want to give a few tips on how to get the two characters that troubled me the most: Flandre and Byakuren.

Also, if you just want the achievement and don't care about an incomplete collection, you can get DLC2 and unlock Miko, which should be quite easy. This way, you can skip one base game character that is giving you a lot of trouble.

For Flandre (Scarlet Devil Mansion 13), the difficulty comes from the fact that this stage requires you to play on Hard or Lunatic while having a high combo and precision requirement AND greatly reducing you timing window. I have the same recommendations for this and passing Re: Unknown X, so please refer to Section 1 for details. Once again, score does not matter here. I would also advise against using a Groove Gauge Rate Up mitama card, as if you need it to pass the stage, you're likely to not meet the mission requirements anyway. You may try using the risky timing increase ones though if you can fill the gauge in time (I haven't tried).

For Byakuren (Myouren Temple 05), the difficulty lies in getting 1000 point items. The best way is to use bombs on danmaku, so try to bring 5 (the max amount) going into the stage. Youmu and Remilia both have 2 bombs, so they are preferred. Do not use Reimu, as she has homing shots that take out the fairies too quickly. I recommend a mitama card that increases damage, as the damage decrease can be very noticeable. Find one that has 2 or 3 bombs. A high level card will also do more damage.
The combination of mitama card and character should get you to 4 or 5 bombs. If you're at 4, get another one from the booster items. I found the most success in bombing the fairies in the first half of the stage: wait until there are a lot of danmaku on screen, then release a bomb. This should easily get you at least 180 points per use. There may be better spots, but I don't really want to do the stage again.
There's always a chance that one of the two item drops will be a bomb, so you don't have to worry about not getting enough in 5 bombs if you don't mind replaying the stage a few times. This is also an advantage of using bombs on the fairies: you will have space to collect the bomb if one drops.

Hope these tips can help you get all achievements with a little less effort! If you have any more suggestions, feel free to comment down below.