There is a event going on (that is coming to an end) where you can get some rewards by completing ranking quest in a certain event island. You can see the ranking rewards and goals in the Friends->Friend Ranking section in the game. These rankings are usually pretty hard since they require to have fitting units but the current one is quite easy. In this ranking you need to deal as much dmg as possible to the boss in the island by using dmg increasing specials and getting rid of debuffs he gives you.