Plus Dust Blast and Poison Spores are mini versions with more limited target counts that can still multiply their damage in the same way. Pyroclastic Eruption can hit for something like 4K per target and if the targets are stacked up, critically hit and all that you can dish out a single spell that deals something in the realm of 100k or more damage.
Geomancer is the one with the single hardest hitting attack, but he about three of them to use.