What to spend crystals on?

When new to the game because you have so many characters to unlock I suggest buying chromiums but after you get a sizeable roster use crystals to replenish cantina and regular energy. When you progress well and are sitting between 51-100 in arena, I suggest using the 50 crystals to replenish after your 5 battles if it means you can get into a better tier before the daily prize. It gives you your 50 crystals that you invested back but more shipments and credits.