For anyone unaware or may have forgotten after all the patches into current retail; old talent trees had a talent you could spec into that made the Blizzard spell reduce enemy movement speed by... either 65% or 75%, it was a massive fucking group slow with a bit of damage. Sure you aim in front of the group so they run and then through it, but that little bit of effort carries a lot of weight when you're dealing with an out of control group; and spell level didn't matter since you could cast every spell of various levels back then. So you'd cast level 1 blizzard for minimum mana but still have it carry that massive debuff, and the same thing with Mana Gems, you cast a few at different levels so you have more than one in reserve.
But as stated above, you hit them with a slow, and you can turn a group problem around, just by letting your team run faster than the mobs... something a nepo baby wouldn't know.