|Ultimate Arena Buddy Corrin is an alternative build meant specifically to maximize both arena score and the effectiveness of his buddy.
Firstly, a couple of points; Firstly, why Corrin? He is a cheap unit so it makes him a good unit to merge +10 for score. Additionally, the legendary Weapon refine Yato has become even more useful with the advent of the additional points for each kill by the bonus unit - boosting your bonus unit +4 in all stats is suddenly very useful! Secondly, In preparation for the arena, make sure Corrin has completed an S-Class support rank with your buddy of choice - this would be one of the weekly bonus units in the arena. This can easily be done by spamming a few GHBs or Training Tower levels and ensuring both units get a kill each time. Corrin can swap over to the next bonus unit whenever you need him.
Skills: Rally Spd/Def+, Aether and Distant Counter are all staples for high arena score. Obviously these are also fairly expensive skills, so try and find high-scoring alternatives such as Brazen Atk/Def 3 if they are unavailable. Rally Spd/Def+ fits in wonderfully with Corrin's pure support role, but a cheaper Rally Spd/Def or similar will suffice. Since no B Skills grant his allies passive bonuses (since he has a rally assist), Corrin takes Poison Strike 3 for the purpose of weakening the foe for his buddy. Additionally, it is also a high-scoring skill. For C, Corrin can take any double spur such as Spur Atk/Def 2 which will stack nicely with his weapon and also scores highly in arena. For the seal, Res Tactic 3 works well on a tactics team (and stacks well with Rally Spd/Def+), or alternatively a drive skill such as Drive Def 2 will stack nicely with his aura. Both these skills score highly.
IVs: Corrin takes a +Res/-Atk IV. This might seem like an odd decision, but the reason for this is two-fold. Firstly, Corrin wants his super-boon in Res so that he can score higher. Secondly, he is unfortunate to have both HP and Def as super-banes so we are left with Atk and Spd. In actual fact this ends up being in our favor - we don't want Corrin killing enemies, we want his buddy to. Give him -Atk for a lesser chance to kill the enemy and instead leave them for his buddy with an additional Poison Strike 3 jab.
Good luck in the Arena!