Sustained AoE will never happen in Mythic+ dungeons. It's either Burst AoE or Cleave/ST nuking depending on the keystone level you are attempting. At M+10 or higher levels, you literally can't pull more than 1 pack, some packs require hard-CC.
For example just today we tried +11 Arcway for lols, our tank got 2-3 shot at every pack, and he is 870+ iLvl(Necrotic doesn't help
). The higher you go in M+, the less you pull at once and the more you do ST with passive cleave(at which Fire Mages and MM Hunters are godly at).