Brotato

The easiest way of doing this achievement is using the Demon. While any regular character use material to buy items at the store, the Demon use HP as currency.

To unlock the Demon, you'll need to complete any level 5. (In order to make things easier, you can go to settings to lower the speed/damage/health of ennemies to 25%, it will make it a breeze and won't void the achievements).

Once the Demon is unlocked, start a level 0 game with him and use only guns, and go through Wave 1 gathering as much materials as you can. Buy everything on the store, reroll the store, buy everything again (and so on). You should be able to target Max HP 1 (if you start a game with Max HP as a negative value, you'll start with 1hp as well so don't worry too much about this).

On the next round, simply stand in the middle of the arena, kill enemies with your guns and don't collect materials on this wave (levelling up will give you an extra HP). You should unlock this at the end of round 2.

I recommend a ranged class like the ranger, picking a gun for your first weapon. Wave 1 starts and run into enemies to take damage until you are at 1hp (i did this with around 4 seconds left) then avoid enemies for the remaining seconds and use your gun to clear anyone that gets close. This Cheevo should be yours

Easiest way to unlock this:

Change the following options in the Options menu:
Gameplay -> Enemy Health 200%
Gameplay -> Manual Aim on
Do this so it's really hard to kill enemies and you can control your aim with the

Then just run into enemies in wave 1 and aim away from them until you're down to 1HP then run away until the round ends.