Mulligans
General Mulligans
Tools to help you survive. Dr Boom is just op, that's why it's a keep sometimes.
Aggro Mulligans
Bad matchup here, don't expect too much. Just want tools that help you survive, mainly aoe clears. Could keep zilliax if you're desperate and the aggro isn't that aggro.
Midrange Mulligans
Mulligan for stuff that can deal with midgame/earlygame stuff. Dr Boom is a keep because it's op, and you can probably afford to keep it without dying.