Normally it is best to try to pull the Archmage into the corridor. This isn't easy though as there are a lot of monsters. Casting darkness helps, as most of them can't see in the dark, and (I believe) the summoners will mostly not summon. Just take it off now and then to check the archmage's position. If you happen to be able to get a point of PP or two, use a wand of far slaying to activate him as soon as you can.
Once he's in the corridor (or at least somewhere fairly convenient), hit him with a potion of confusion or wonder. This will prevent him from casting spells. If you're good with missiles, humanoid or daemon slayers will help. Otherwise, go in for the kill with the Rune Covered Trident or another daemon/humanoid slaying weapon if you have something better. If he starts casting again, reapply confusion right away. Hand-to-hand combat with him is safe enough if he's confused--he can hit pretty hard, but nothing potion of extra healing can't take care of.
Oh, and the amulet you're talking about with [+4,+4] {+7 Wi} is Preserver. It is much better than the ankh.
[edit]For divine casters like priests and druids, Rain of Sorrows (acid ball) is definitely worth a wish as they have a very hard time finding the spell otherwise.
Hoping to win with every class, doomed. Archer, Barbarian, Bard, Beastfighter, Druid, Elementalist, Farmer, Fighter, Monk, and ULE Priest down.