Raiding feels like work; I only raid to fill the chest. When I get an elemental chest in place of the hero chest, I rejoice that I am not going to raid that day. Make it more fun and reduce the frustration.
FUN: making combos, using my heros specials, winning.
FRUSTRATING: “ghosting” tiles, heros getting killed before they can attack, losing.
I suggest cutting the rate defenders gain mana and/or reducing or eliminating defender’s non-special attacks to give a signifacant advantage to the attacker. Since the attacker is playing while the defender is off-line, the attacker’s enjoyment should be paramount.