|For when you want Clair to tank more than just mages. Using a Def-refined weapon as well as her +Def superboon, Clair can raise her overall bulk to the level of lance fliers who are more well-known for their tanking ability, such as Tana and Catria. Since Tana and Catria both have superboons in Res and lower or equal Res than Def, assume all three fliers try to patch their lower defensive stat with their superboons and (if possible) Def/Res weapon refines. After considering weapon effects and Close Defense, each flier ends up with:
Clair (Rhomphaia): 36 Spd, 44 Def, 45 Res
Clair (Tannenboom!): 38 Spd, 46 Def, 47 Res
Tana (Vidofnir, Res boon): 36 Spd, 44 Def, 41 Res
Tana (Tannenboom!): 38 Spd, 43 Def, 43 Res
Catria (Tannenboom!): 36 Spd, 47 Def, 43 Res
Catria (Berkut's Lance): 34 Spd, 45 Def, 48 Res
So, out of the three, Tannenboom! Clair does the best in terms of sheer defensive stats, followed by Berkut's Lance Catria and Rhomphaia Clair.
The choice between Rhomphaia and Tannenboom! ultimately comes down to preference: Rhomphaia is budget and has dual effectiveness, while Tannenboom! further improves her Def/Res. The Spd gained from Tannenboom! is also useful, as you can avoid more doubles from Heroes with Def-ignoring abilities like Leif and Karla, and even reduce the amount of damage you take from Karla. Overall, Rhomphaia is more useful in Arena and Arena Assault, due to both modes being overrun with armored units at high tiers, while Tannenboom!+ works better elsewhere, in order to handle Infantry and Fliers better.
Because of Close Def stacking in the A and S slot, one of the few ways most non-green melee units can significantly harm Clair is through Specials. As such, Guard is used to prevent that from happening. The C slot is mainly flexible, and you can use it either to boost her defenses (through Atk debuffing skills or Def/Res Waves) or support her teammates (through Drives, Flier buffs, Atk/Spd waves, Guidance, and Tactics).
Clair with this build can consistently take 0-2 damage from non-green Arena threats like Hardin and Female Grima, and can even tank green units like Male Grima and take < 5 damage, provided she is on a defense tile. Combined with a ranged tank of a different color to take care of archers and mages, such as Distant Def 9 Faye, a Raven tome user like Lyon, or a Distant Counter unit with decent mixed bulk such as Dorcas, she can become nearly unstoppable.
Note: All matchup and stat calculations assume +0 units.