I like hard mods as well (harder than the original game at least, since it was far too easy), but I've just played the 'Testament' level and I have to say
the 'sea of flesh' is simply impossible to beat, with my character setup/equipment at least (longsword guy, battle axe minotaur, short bow minotaur, mage insect, 4 health potions, 2 energy potions, mostly rather crappy armor, two bombs). And when I say impossible, I mean impossible. I will gladly send my savegame to anyone who wants to try and prove me wrong. Waves 1-3 were easy, in wave 4 I started to lose a bit of health, in wave 5 I needed my health potions already and in wave 6 I was surrounded by spiders who killed what was left of my party in a relatively short time (could kill two or three of them, then I was done). And then there is wave 7, which is even harder than wave 6. In the end I had to cheat. When I saw what happens afterwards I was glad I did, because then you face even more tough enemies in tiny rooms with traps shooting at you all the time - and you don't even have a healing crystal to heal any dead party members. It is too much combat for my taste and, like I said, impossible. Probably if I would have found more secrets (=better equipment, more health potions, possibly a character level or two more etc) it still would have been hard, but at least doable I guess. To me the fine line between challenging and frustrating was crossed here by a good margin.
Personally I found the 'path of the dead' far,
far easier - it was certainly challenging, but not too hard, I didn't even have to think about cheating. Anyway, will keep playing now since this seemed to be the hardest part of the mod when I look at the comments. And I really liked level 3, so I hope I will enjoy the next few levels just as much.