Instructor

Instructor is a low-rarity set, but its four-piece bonus still has a job late in the game. The two-piece bonus adds Elemental Mastery to the wearer. The four-piece bonus gives every party member a large Elemental Mastery boost for a few seconds whenever the wearer triggers an Elemental Reaction. That makes it a cheap way to help reaction-based teams, because the wearer only needs to trigger the reaction, not deal much damage.

It is easy to farm and its pieces are low rarity, so the wearer's own stats will be weaker than with a five-star set. That trade-off is fine on a support who is there for the buff. On a character who deals the team's damage, pick a five-star set instead.