Warning: geeky notes about the Diablo II game (and nothing else) follow:
If you play as a necromancer, cast 'iron maiden' on your summons and Duriel sort of kills himself or at least whittles his hit points down. Keep recasting clay golem right on him (lots of blue potions in your belt if needed) and immediately cast iron maiden on it again each time. Make sure you have a skill point in golem mastery.
Put a portal scroll on hot key and open one as soon as you enter his chamber --- if things go south, just exit rather than getting killed.
Having decent gear is important, as is just 'grinding' a bit before going in to level up.
One particular grinding thing if you are close to level 27/28 is to get an 'insight' runeword polearm for your act II merc --- especially important if you have a mana-hungry character but 'insight' is a good merc weapon at that point regardless.
Taking a stamina potion just before you go in can allow you to do a sort of 'parthian shot' thing where you just keep running away and shooting at him where that applies, though in that space he can still get shots in on you. Having the right potions queued up in your belt doesn't hurt either with hot keys to take the appropriate one as needed or to feed one to your merenary to keep him alive (and do keep your merc alive if at all possible). I suggest you go with an act II mercenary; the act I rogue type is more fragile, harder to keep alive in there, and an extra aura can help.
Merc aura: 'prayer' isn't fast enough to help you against Duriel, so blessed aim or defiance depending on whether and by how much increased attack rating for physical attacks is needed. Noting, though, that if you expect to be running constantly away in circles, defiance does no good --- defense is basically zero regardless if you're running, only useful when standing still or walking. So either run and keep away from him --- likely losing your mercenary in the process, or set your run/walk speed to 'walk' and go for a good defense and possibly shield block to help stay alive.
Best way to deal with this fast moving monster in a small space is going to vary by character class and build design. A sorceress, for example, should absolutely static field him down to a quarter of his original hit points.
If you ever make it past Duriel in 'normal' play, you can later collect better gear that future starting character scan use to make it a lot easier when facing him. Until then, if you can find or buy 'normal' (not magic) 2-socket armor, 'grind' until you collect one each 'tal' and 'eth' runes and make 'stealth' runeword armor. Even better, make 2 sets and put one on your mercenary.
Sorry to ramble on, that was sort of a "
Pavlov's dog hearing the bell" thing for me.
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