Summoned a -Atk +Spd Fallen Robin build suggestions?

The extra speed can definitely help him avoid getting doubled, and his attack is so high, that I wouldn't let it bother you. also, the dragon emblem buffs should negate the lower attack

Well, he still has 53 attack and the speed boon combined with Fury(in which case he would have 56 attack)can help him avoid quite a lot of doubles that he wouldn't be able to otherwise(30 speed), and a Hone Dragons/Hone Speed buff allows him to reach pretty decent numbers for a unit that is supposed to be slow(mainly because -spd is usually the best bane). Aether could help a lot with the after-combat damage from Fury, and you can charge it really fast with Vengeful Fighter(even proc it on the same battle if you use Quickened Pulse seal), but Ignis or Bonfire(because you won't get doubled as much as expected if you invest into speed) seem good choices as well.

Congrats on pulling him btw!