You gotta use thawing potions. Duriel's damage is mostly cold, from his aura. So drinking thawing pots, negates a lot of that damage. Both for you and merc, though merc takes a lot more physical damage due to the game's inner mechanics.
But yes. As to weapon - what skill are you using? If you use like concentrate, level 18 skill, shout to increase defense, and war cry, to negate duriel's defense, it should not be impossible to stand there and swing, even if you are chilled from his aura.
But yes, gambling is an option. Get a good rare weapon (1/10 chance for rare), of the weapon type you are focusing on. Otherwise, a 2-hand polearm, the low-end choice is 3 sockets with gems, should be what you need.