Mulligan until you get Erratic Explosion and don't have all four Gleemaxes in your opening hand. If you are playing against 2-4 opponents, use Kaboom! instead, and make sure you have at least the same number of Gleemaxes remaining in your deck as the number of opponents.

Play down a mountain every turn until turn three (or five), then cast Erratic Explosion, targeting your opponent. This will reveal Gleemax and deal 1,000,000 damage to the named opponent.


EDIT:

The best deck is now officially in the best format.

With the printing of Calibrated Blast, we can adapt the deck to modern by swapping Gleemax out for 4x Autochthon Wurm. Blast also allows us to win at instant speed, and gives us a second chance if the first spell didn't deal lethal or if it was countered.