Try the new AI-powered Infinite Story.

Swgoh farming flowchart

Much like the Guild Store, the Fleet Arena Store rotates what shards are available every 6 hours. Also like the Guild Store, it's best to focus on 2 units at a time until they're both done, before investing Tokens elsewhere. Focusing like this will complete key squads faster, netting you better results in-game at a quicker pace.