Complete all levels in co-op with another player(s). This can be done local or online. Play all levels in the main game (Act 1 - Act 4, 5 levels each = 20 levels). The game doesn't keep track of which levels you played in co-op, so you'll have to keep track yourself. If your leveled up, completing levels goes faster. This applies to you and your partner, but you don't need to be leveled up, it justs makes it easier. Happy Gaming!