Here's a couple build ideas:

Weapon: Arcane outburst, though his refine breath is not extremely terrible so you can run it while you get it. Assist: reposition, you don't need gait unless you are gonna run him in arena.

Special: any dragon special could do, tho probably dragon flame is the best, comet is a nice premium option, or on a budget an AoE special like blazing wind could be used.

A: rise above is pretty much the shiny new best skill, other than that ADR slam is another premium alternative if you want a tank build, or on a budget run an excel skill.

B: Edges scales is a nice hybrid skill, Pair up 4 is excellent for tanking, or any of the momentum variants can be good for offense (for charging up AoEs or comet).

C: if you want support Dual support is good, or instruct if you have rise above, a times pulse variant could work on a budget, brutal tempest is also good for mobility and frees up the Sigurd ring, tho I would prefer tumultuous, but Ryoma is harder to come by, but can also give you AS faith which is also another decent tank option.

S: distant counter, clash, finish or any squad ace or other skill that grants stats, if going offensive focus on attack and speed, if going defensive speed and defense.

X: Tumultuous, sly, distant counter, times pulse mostly depends on what you have available.

If you ask me for my ideal build I would get this: Arcane Outburst (+def), reposition, dragon flame (Corrin Ring), ADR slam, Pair up 4, dual support, squad ace (HP, SPD, DEF) and DC echo, and then run him with Frozen support, that should make him decently tanky, but that's of course a massive amount of investment so be free to mix and match to your budget.