Your focus should be on getting your start champ up to level 60 by running campaigns over and over.  Brutal stage 12, floor 3 (often seen as Brutal 12-3) is the best Exp-silver per energy.  Once your starter is strong enough to win that stage by him/herself, bring in other champs that you don't need to level up the champs you do need.

Just grinding through campaign works the best, especially when you get to farming 12-3 or 12-6 Brutal.

edit: snailed