So, can a melee/firearms whatever type build kill Dredmor?
Crossbow types can, sure. Fire all of your BoMD, then your Squid Bolts, then just plunk away whenever you get a chance.
Throwers probably can do the same if they invest in the new clockwork sawblades.
Priority is on staying alive via keeping up healing and resists. Also consider wielding two Mirror Shields or Mirror Darkly Shields, to reflect all his ray spells. Be advised that his electric attack can't be reflected, so you need to be able to tank it. If you're not ready, he can one-shot you with it.
If you can silence him, things are easier. Magic Training and Magical Law have silence spells, and I think Clockwork Knight?. Also the Sonic Wand, but I'm not sure I'd trust that, it's a beam so it might bounce off him.
Melee is harder, as far as I know. He supposedly hits
hard. Concentrate on getting as much damage reduction (white armor icon) as you can, or raising Dodge (yellow swish icon) way over 100. On attack damage types, go for high core damage types instead of exotic types, as he resists almost everything. Exception: he's weak against righteous damage.
Dredmorpedia says Dredmor has:
20 asphyxiative, 10 conflagratory, 15 existential, 10 hyperborean, 20 necromantic, 5 piercing, 10 putrefying, 20 toxic, 10 transmutative, and 10 voltaic. That's on top of 20 armour absorption, 58 block, 37 dodge, and 27 counter.
The 'pedia also has stats for a "Tougher Lord Dredmor" who looks
really scary.