Lyon B skill

Swordbreaker. QR you can put it as Sacred Seal (asid from the fact that if attacked meelee he won't make good use of it). And i'll tell you, Raventome + TA is hell on earth for archers. My Robin (M) takes 0 damage from a +10 merged +Atk Brave Lyn. And he has only 30 def

From someone who runs Sanaki with TA/-raven/Swordbreaker: Do it. This build is the best thing that’s happened to me in this game XD

Greens and colorless both get annihilated, sword-breaker make it so swords stand even less of a chance. And like Remnant said, QR you can grant with the Seal, if you got it.

I'd say QR seal is the best seal option for him, leaving your choice of breaker skill for B. Axebreaker if you find that he falls just short on a few matchups, otherwise probably swordbreaker, maybe bowbreaker if you're having trouble with merged Brave Lyns or Fortify Cav buffed Brave Lyns, and need that QR seal on someone else for AA. An unmerged Brave Lyn with only Hone Cav on does get one-shot by TA Lyon on counterattack. so yeah, TA Naglfar should do really well. Find the niche you want him to fill, be it a broad but not as deep coverage with swordbreaker, non-blue ranged coverage with QR, or focused with a little extra depth with Axebreaker or Bowbreaker. It's really a matter of priorities with the rest of your units. If you just want a Brave Lyn and G tome check, QR is a good idea. If you need more breadth, swordbreaker is great, but you might fall just short of where you need to be in certain matchups against merged units. When we get the chance to +10 him eventually, those won't be an issue and you can go swordbreaker for sure. Just take your pick for now.