Mulligans
General Mulligans
Aggro things. Kill them quick with keleseth buffed huge minions.
Aggro Mulligans
Contest the board with weapons, spells, freeze effects, and keleseth buffs. Play Saronite Chaingang because that is bound to slow them down a little bit.
Control Mulligans
If you didn't kill your opponent by turn 10 it might look like you've lost, but you always have the combo if you haven't played Leeroy yet. But it's always better to try to kill them before then.
On turn 10, if your opponent has survived that long, play leeroy, hit face, then cube it and hit face with your necrium blade, breaking it and triggering the cube, giving you two more leeroy going face for another 12 for a total of a 21 damage burst combo. 22-24 depending on if you played your keleseth a few times or not. If you didn’t, you might shadowstep the leeroy back after that to give it another go the next turn.