Best build for +atk/-res boey???

That build seems good, that’s generally what boey’s used for

Sure. Looks fine. You probably don't need Deflect Missile seal. Check to see Deflect Magic can help you survive Reinhardt.

Do you just happen to have a 5* +Atk/-Res Boey? I wouldn't take a super bane on Res if you are building him to +10.

Also be cautious of investing 20k feather into his weapon. If you need a Lyn counter for AA, just use a 4* raven tome.

I'd opt for slightly different skills.

Gronraven+, no arguments here. On my own Boey I use Gronowl+ more often but that's because I killed a Takumi.

Ignis, It's a fine skill and it's his default. I prefer Bonfire just to use more often but it's personal preference.

TA, Raven + TA is the way to go. Again on mine I use Close Counter quite often but when I do use Raven tome it's TA all the way.

Bowbreaker, too strict. Boey will laugh off any archer anyway, who cares if they double you? Quick Repose (even just QP2) is much more versatile. With QP you'll still hit back twice to archers, but also gain an extra hit against blue mages and daggers to secure kills.

Threaten Res, great skill for him. Anyone who survived the counterattack will take -7 Res. Again, on mine to wall off better I use Threaten Attack but Res is great too.

Deflect Missile, maybe? It seems overkill, especially with your initial bowbreaker. But if you take QP and still struggle with quad archers it'll buy some extra safety. If you keep bowbreaker it's a definate no (since archers won't double it'd only help with 1 hit from a brave bow), if you take QP there still might be better options.

Boey is a great unit if built well and you have good IVs. If yours is 4* and you don't plan on using a Close Counter build you can always save feathers and make a 4* merge.