More information would be helpful, like you build and gear.
I killed Tchort with melee builds. Hammer, knife, unarmed - all of them. Didn't even had to destroy the tanks or solve the puzzle, if i remember correctly.
A try to destroy a mutagen tank <--- downright impossible, I can get max 1-2 hits before tentacles destroying m
Try using TNT charges. There are some lying in the warehouse area. One charge is enough to destroy the tank.
B going directly to the boss, he has like 20% of his health left before i'm killed, my hypos doesn't help me much.
Try using drugs to help you. For example morphine + lifting belt + mushroom brew and you are VERY resistant to damage for 3 turns. Stuff like focus stim, giving +15% crit chance, will increase your overall damage immensely.
C mutangen puzzle, I gave up this is just too boring also only have like 8 mutagens
The puzzle is not that hard. If you have troubles, use this link to help you
http://underrail.info.tm/mutagens/. You can even just ask for help solving it on forums and steam discussions, a lot of people already did. If you don't want to spend time looking hard for mutagens, you can look up their locations on wiki.
Styg said something on the forums "you don't need mutagen puzzle if you made i through the caverns" sorry man but that's bullshit; I made it trhough the caverns only to be fucking destroyed by the last boss.
You really don't. Maybe not you specifically, but if one overcame challenges of the caverns (like how to deal with high mech resist, for example) then one can kill the last boss. Try varying your tactics and general approach to battle.