Honestly it's kinda useless reaching max lvl on all the bosses it just gives you 3-4 more mythic cards everytime you collect after completing it all..........not worth the bother to install and try diff mods for this game like i did
Every starter deck has 1 card skill that x2 damage of the element girls for 3 turns. Use that skill whenever it avaible and you can clear any lv25 easily