I would see it this way:
- This level should be easier for berserks. We only have one true berserker level.
'Berserkers', that is, marines presumably choosing at least a few melee traits, already have an entire string of special levels that benefit them specifically - Chained Court, Unholy Cathedral, and the Mortuary. 'Because they don't have enough' isn't a good enough reason on its own to give them something else.
- No diagonal movement!
Why? What would the problem be? Nethack handles it well enough, and levels can be/are simply designed so that you can only move the rocks in the cardinal directions, anyway.
- Lava barrels. If you shot, lava spurs out, sometimes blocking the puzzles. Tough luck! You still can exit the level normal way.
Sokoban, at its heart, is not about combat. While DoomRL is, I honestly don't mind the idea of a special level that doesn't involve combat at all, like 100% of the rest of the game. Branching from that, my point: without combat, you wouldn't shoot any of the barrels. On the other hand, if there
was fighting involved on this level, as can be the case in Nethack, this kind of random 'screw you' doesn't look good on a level's resume, similar to how you were once able to destroy the lever in Hell's Armory.
- As for the enemy I would imagine some great daemon that throws barrels at you or pieces of wall or corpses, whatever it has at his hands. If you get hit, you get pushed back, making it harder for you to reach it. This daemon is very slow, but can kill you in two hits. Yup! But if you are a fast berserker/dodger you can use the hit and run tactics. As for ranged weapons, it is heavily armored and it's hard to do any damage from range unless you hit its weak spot (bonus from Eagle Eye perhaps?)
'Moar damage' and 'moar armor' aren't particularly cerebral answers to the question 'how do we make this boss interesting and challenging?'. It seems to me that, after solving a brain teaser, an encounter with a 'boss' should be creative and inventive, not 'hur dur kill you in two hits while not taking damage'. (It could be argued that, after all of that thinking, Doomguy wants some action, but we already have several big, heavily-armored, highly-damaging enemies to fight... they're the reason you went to Hell in the first place.)
I plain don't like the idea of making it 'for berserkers', either. Since it's Nethack's Sokoban that spawned this idea, maybe one of its artifacts are given? A bag of holding, granting a few extra inventory slots, or an amulet of reflection... its original function would be absurdly overpowered in DoomRL, so change it to a percentage chance, and/or perhaps upon being hit by a projectile.