I would honestly just focus on getting 95' Mai. Purple Boss units are a pain in the ass to level up, plus you still have to get their elusive fighter unit. Anyone in the default banner is easier to level up due to being able to exchange soul dust for their soul cards every day, and some have soul events in the story mode (normal, hard, and expert) that you can do 3 times a day for 3 soul cards. Also you can save up league tokens to get a selector ticket for special cards of anyone in the default banner, something you can't do for the boss banners or other banners.
Long story short, if you're just looking for a good red unit, focus on getting 95' Mai (red attacker).
Also, if you finish the Yellow Element Festival, you gain access to the Red Element Festival, which has you unlock a free 5* Vice, who is decent apparently.
ugh that involves so much investment in yellow terry though :|