Garthanos
Arcadian Knight
That too though crit being infrequent enough ... there are however way more methods to boost it.Note that in 4e, all spells were attack rolls and therefore could crit. In addition, magic items gave you bonus critical damage dice per plus.
Then the feat to increase the area effect and reduce damage suits that goal well.Strange, I've always seen the 4ed Fireball as a way to kill minions. Nothing else.