There's a bunch of discussion about good spheres here.
http://forums.civfanatics.com/showthread.php?t=300800
Stack-blasting archmages are pretty much a thing of the past, they used to rule lategame battlefields but were then nerfed. There's 2 arcane anti-stack spells: Maelstrom (Air2) and Wither (Entropy3). Both have much lower damage caps than the old anti-stack spells archmages had, though wither's pretty mean anyway due to its secondary effects (no healing, halved experienced gain, -10% base strength)
You want stack-blasting, you go divine. These are your options:
Ritualists (Ashen Veil priests, need priesthood)
Cultists (Octopus overlords priests, need priesthood)
Chalid (Empyrean hero, needs religious law)
Luriduses (Empyrean high priests, requires Theology and high-leveled vicars. Vicars you get at priesthood)
High priests of winter (Illian unit, needs Theology and high-leveled priests of winter. You get 3 priests of winter through a ritual available at philosophy)
Dwarven Druids (Khazad/Luchuirp unit, need commune with nature)
Fireball golems (...kinda. They have limits on how much of a stack they can hurt, but the sheer NUMBER of fireballs compensates. Luchuirp unit, requres sorcery, fire mana, and the blasting workshop building. Okay, this one's arcane, not divine)