This one is REALLY hard, as there are only 3 people who have got it (on this site anyway); myself, the guide poster above me, and a cheater.
If you didn't know what this achievement is, you gotta beat the game on MADNESS, which is unlocked by beating the game on any other difficulty. The above guide is very helpful (i used it towards the end) but i'd like to add a few minor tips.
- PLAY SOLO! I tryed coop in this game and nothing went right. You may think it's easier, but its not, you'll just get dragged down.
- If you don't have all weapons at level 3 DO NOT ATTEMPT THIS. You will be overwhelmed in no time. You don't need every one at level 3, but the MINIGUN and FLASHLIGHT must be, i used these throughout the game.
They burn ammo really fast, but I found nothing quite as effective. If it helps my hotkeys were the flashlight for UP, cd launcher for RIGHT, rocket launcher (which I never used) for DOWN, and minigun for LEFT.
- Break everything! If you got all level 3 weapons you'll probably have a lot of tokens to spend, but the machines aren't readily available always. You'll get Jolts or full ammo boxes from crates, vases, etc... and you'll need em.
Lastly, don't worry about dying so much, because you will... A LOT. If you're going for the suicidal task of S ranking every madness level, you have to beat them all first.
As for specific moments where I had a large amount of trouble, they were few except for the final boss which there are many tips above for that.
So... that's all I can say. Feel free to message me here or on live for any help... and if you were wondering I have 10/18 madness levels with an S ranking :P