if you got trouble with immunes and your merc cant take on one of the ancients 1vs1, then go for a hybrid build. something like orb/hydra, blizzard/hydra. meteor or fireball works aswell. 20 into orb + cold mastery until you hit ~ -150 to -175. 1 into static, teleport, warmth, rest into some fire spell, their synergy and fire mastery
the way i usually do it when playing through the game, wether with nova or blizzard is to reroll the ancients to get a good roll (only 1 cold/light immune that doesnt have stoneskin) - lead them all to the top and try to seperate them so you can 1v1 each, then let your a2 merc kill the remaining immune one. if you got an armor or a helmet that has physical dmg reduction %, like shaftstop, that'll make your merc way beefier vs them.
also fill up your inventory with mana and health potions and drop them in the ancients arena if the figtht will take long. your merc will need some dmg for that tho, if you dont have a good weapon for him or he just dies all the time go for a hybrid build