Any way to fix a -atk +res S!Ryoma?

29 attack with 34 speed is pretty bad
And res is still non existent so, just some LnD build if you really wanna use him
If I was you I would just fodder him off after the Arena ends for chill def or his dagger

Just use the normal suggested builds since his most important stat is Spd. Don't get to focused on Ivs, if you wanna use a Unit, you can. This game is not a competetive environment, so you really should not care about Ivs as a f2p. But if you pull another copy with better Ivs for example on a legendary Banner you can merge them.

Nope, offensive stat line is poop and defensive stat line isn’t helping him. At least you could fodder his dagger to someone.

There are plenty of ways to fix him, and he is by no means unusable. He's a dagger unit, so that alone makes him have some type of usage. Not only that, but his IVs hardly even matter unless you were planning on max-merging him, so I can't really see anyone using him for Arena in the long-term unless they had a few merges under their belt.

He hits 44 attack and 42 speed with a speed-refined dagger, Fury 3, and a Speed +3 seal. 46 speed if receiving a Hone Spd buff (it gives +4 speed right?).

Not only that, but his main priority is debuffing, like almost every other dagger unit in the game. He gives a -7 Def/Res AoE debuff to the enemy after he attacks. You can either keep that or give him Smoke Dagger to debuff all 4 stats on the enemy, or Rogue Dagger to buff your ally's defenses as well as debuffing the enemy.

He may have 'one of the worst IV sets' but he's completely fine in terms of usage.