This is the game's hard mode. It's not too difficult due to your healing factor, but certain enemies do take a while longer to drop compared to Veteran mode. The only real tough spots are on the last stage where you must fight your way through tons of enemies to get to the last fights, then when you have to fight all three Marauders at once. Recommended to playthrough on normal first to upgrade your skills to make this much easier. If you've already played through on normal and just need this achievement, you can teleport through a lot of the stage space and past many of the enemies. Enemies will eventually catch up to you and usually swarm you, but by then you'll have hit a checkpoint or two and won't be far away when you respawn. Your guns are your best friends here. Once upgraded, you can use the counter attack with the plasma rifle on melee enemies and many will die in 1 shot. This also doesn't use your ammo since it's a counter. For the ranged guys, just switch to your SMG and they drop fast. You also don't have to do these levels in order for the achievement to pop. I did the first few missions, then the last 2, then the ones in the middle that I hadn't done and it popped after I cleared the "Death, Baby" stage since that was the last one I hadn't beaten on Ultra-Violence mode yet.