I went for all the fire rate and dmg with defensive buffs, the build was so good it was 2-3 shotting the bigger enemies as well. Plus the fire grenade is op.
(Tip: Tap & hold on the T dolls to view their attributes when purchasing after a Wave)
Beware cuz some dolls have negative effects too, like magician classes drain hp per shot for some extra damage so don't just purchase random T dolls