How about we do away with power attacks and unlocks all together. Instead, allow unlimited helper slots for boss battles. When a boss battle starts the players will begin with a 10% att/def handicap against the boss. Each player that joins will reduce the handicap by %1 after 10 players join then all players involved will receive a %1 att/def boost against the boss. Rewards still will be given based on speed of boss kills and damage done.
