This guide isn't perfect - there may be faster ways to farm for money / levels but this just demonstrates the methods I found most effective
It also doesn't include the paid content so as to be more accessible for the majority of people. If you have the bonus maps and have strategies for them that are better than mine, leave them in the comments

Initial points
If you see any special units while going for any other achievements, kill them. They will almost definitely be the longest and hardest to get so work on them throughout the grind

How to farm money
Low Level money farm
High Level money farm
Kills / money farm (high level) - just complete the robbery as usual but if you see any special units then kill them to work towards their respective achievements. If you already have the plundering prodigy achievement (steal 20,000,000) then you can meet the minimum take with only the duffle bag full of money at the back right of the vault
If all you need is kills then you can just destroy all 4 powerboxes (actively destroying a power box causes more units to arrive) and then you can restart the heist

How to farm experience
Experience farm (any level) - while you can just shoot the cases, I don't believe you get nearly as much experience for doing so

Complete a heist for the first time.
Just complete the jewellery store heist loud or stealth, it doesn't matter

Kill a rat
They are outside the buildings and you can just shoot them with any of your guns

Steal everything valuable during a heist
I recommend doing the jewellery store robbery
Take everything from the vault and the stack of money on the second floor

Complete a heist only leaving with the minimum take
The minimum take can be viewed in the truck
You do not need to match the number exactly by just ensure that you take just enough to meet the take but not anything extra

Complete any robbery without activating the alarm, open the vault and then activate the alarm by being seen by a guard or camera - any other way of activating the alarm works too

Complete a heist without equipping guns or mask
I recommend this video by Baked Bread as it requires only the default lockpick

Wait for an interceptor unit to arrive, let them place down some barbed wire and then walk through it

Wait for a blinding shield unit to arrive and just stare at the shield while the lights are on

Blow yourself up with C4
Look at the tips section for how to farm money and experience to unlock the C4

Complete 50 heists loud
I completed Liberty Hawk Bank stealth (taking around 2 mins) and got caught before leaving to make the heist loud, you can just shoot a weapon with no silencer to alert the guards
Low level route
High level route

Complete 50 heists stealth
I completed Liberty Hawk Bank stealth (taking around 2 mins) over and over
Low level route
High level route
You will want to bring a gun with a silencer so you can shoot the cameras without alerting more than one guard

Pick 1000 locks
Follow the route for farming experience in the tips section

Steal $1,000,000 total
Steal $5,000,000 total
Steal $20,000,000 total

For efficiency combine this with other achievements like loud / stealth heists and farm special unit kills as you go

To farm money I recommend one of these methods
Low Level money farm
High Level money farm
Kills / money farm (high level) - just complete the robbery as usual but if you see any special units then kill them to work towards their respective achievements

Kill 250 police
Kill 1000 police
Kill 5000 police

You should get this while getting all other achievements, if you somehow don't just reset orbital bank and kill them as they come to you

Kill 250 guards
Kill 500 guards
Kill 1000 guards

You should get this whilst going for the rest of the achievements but if not just start the orbital bank heist, kill all the guards and restart the heist

Kill a powerbox defuser unit whilst they are turning off a powerbox
Kill 50 powerbox defuser units
Kill 100 powerbox defuser units
Kill 300 powerbox defuser units

Use this guide to farm kills

Kill 50 interceptor units
Kill 100 interceptor units
Kill 300 interceptor units

Use this guide to farm kills

Kill 50 blinding shield units
Kill 100 blinding shield units
Kill 300 blinding shield units

Use this guide to farm kills

Kill 50 shield units
Kill 100 shield units
Kill 300 shield units

Use this guide to farm kills

Kill 50 juggernaut units
Kill 100 juggernaut units
Kill 300 juggernaut units

Use this guide to farm kills