One other thing I have been considering, but did not say on chat, would be to also damage yourself, but increase the damage done to the opponent.
For example (let's say full damage)
Unupped
You receive X/2 damage, opponent receives 1.5X damage - so in case of 25 creatures you get 13, opponent gets 38. For 6 creatures (the average I believe) 3 damage to you, 9 to opponent
Upped, you receive X, opponent 1.75X - you get 25 damage, opponent 44. With 6 creatures 6 damage to you, 11 to opponent
Too much?