Units are the game's premium currency. The more you have, the better your chance at obtaining rare champions. Save Units and spend 100 of them on the Premium Hero Crystals. You can get 2 to 4 star champions from them. Play and win Versus mode matches to get Arena crystals that can be spent in the vault for a chance at obtaining a 4 star hero. You will also win Battle Chips that can be used to obtain a good hero or free Units.