The Warhammer Boltgun: Forges of Corruption expansion on medium.
It was on the tougher side for most of the game, but the last level was fucking bullshit. On the last level, you have to fight multiple waves of enemies. The problem is there's so much shit flying at you, the game spawns near infinite waves of TANKY enemies, and even though I MAIN FPS games and have played them for 20 YEARS, I could not do SHIT when the game starts spawning 50 tanky enemies and like 10+ bosses from the prior levels. I have even beat the original Far Cry (which has notorious hitscan enemies and the hiding mechanics are completely broken on newer PCs, making everything and everyone shoot at you from across the map) on medium WITHOUT cheats.
So I did what I have only done one other time to beat a game. I downloaded Cheat Engine, found a cheat table, and God Moded myself to beat it. I do not like games that are too challenging to the point where they make me want to punch my monitor. Haven't been there yet, though.
Looking back, I feel like I probably softlocked myself from taking health too fast (would use the healthpacks whenever I got to 50%-ish, but it shouldn't have been a problem to begin with tbh.