Do cave, if you want magamatas and medium refines then save up for lucky board and reedem all your points for refine items.

Focus on battle armor from the monthly event

Charms you get from Ult training and sometimes from points reedem. To get a lot of power buff from charms, you'd have to spend a lot.

Save up all types of refine runes until a refine event to get to a very high level of refinement.