I've been using knives and crossbows, specializing in stealth, special bolts and status effects. I didn't follow any pre-made build but I did plan mine out. You can see it
here if you wish.
I'm level 25 when entering the Tchort fight. I fare well in combat. I won the arena ages ago, and I can kill all sorts of raiders and random encounters, even death stalkers and tchortlings in the deep caverns without an issue.
But I can't kill Tchort. Neither can Tchort kill me. I walked in with 4 TNT's (got lucky, didn't know it was a thing), destroyed the tanks with stealth. Then I spent
3 REAL LIFE HOURS exchanging blows with Tchort. He's immune to bleeding, immune to status effects, and my low-dmg weaponry which does just fine against human, animal and robot enemies found elsewhere in the game barely offsets the damage he regens. I used over 500 bolts, 50 bandages and countless batteries with nothing gained. It's not difficult to stay alive or to do damage, but the numbers just won't work out. I ran out of bolts before he ran out of HP.
Now I know the correct solution is to load an earlier save and do the Mutagen puzzle, but it really sucks having to resort to checking the wiki to find a clue how to advance, when so far the game has done nothing but reward and reinforce that the build I've been using works and is viable. And it is viable. Viable against every single enemy in the game except for the final boss. After 100 hours it feels a little cheap losing to an enemy just because they have a bunch of "immunities" to every tool in my toolbox. Well, technically I didn't even lose. I can stay alive indefinitely. But I can't win either.