|Despite boasting beautiful mixed bulk and one of the highest BSTs of all infantry units, I've seen Libra come under fire for his lack of min-maxed stats. Of course, who needs min-maxing when you can utilize it all?
To begin, Libra's average speed stat needs to bolstered to truly make use of this build. A speed boon should be taken. As for a bane, it doesn't matter so long as it isn't in attack. That said, he will wind up with a superbane regardless. Though, it is important to note that whichever bane you take, you should refine his weapon in that stat to make up for it. In the case of an HP bane, it can really go any which way with the refine, though speed may bolster the niche of this build.
Libra's weapon is incredibly important to this build. He needs to have a weapon from Winter's Envoy. Thankfully both the Handbell+ and Sack O' Gifts+ are identical in effect, meaning that either will suffice given you have the fodder. Refer to the above in regards to the refine.
His assist and special are flexible. Given how all of his stats will be roughly the same, the special is almost entirely up to your discretion. Likewise, his assist can go any which way, though Reposition or Swap may yield the best results overall.
Looking at his A skill, you may see where this is going. Kestrel Stance, combined with his weapon, grants him +6 Atk/Spd on enemy phase (as well as +2 Def/Res). This means he has 55 Atk and 41 Spd. He'll be difficult to double for any unit, requiring 46 Spd to do so. You may push this even further by opting for the Spd refine on his weapon. His attack will also be extremely high, and this is where you ought to think about his special. Choosing a healing recovery like Sol may help his overall longevity, though an offensive one can kill foes before they can deal lasting damage. Choose wisely!
His B skill and Sacred Seal are where things get REALLY dirty. Windsweep and Phantom Spd need no explanation. At 35 Spd (given a boon without the refine), he can nullify counters from most melee units, as they need at least 45 Spd to do so. Like with enemy phase, a Spd refine can help prevent even more counters (requiring 48 Spd.) Though do take caution, as units who can overcome Windsweep can also overcome Handbell/Kestrel Stance's speed bonuses, and those who can do so most easily are sword units (Mia, Karla, L!Ryoma, and Lon'qu, for instance.)
His C skill raises his defensive properties further. Even Spd Wave puts his speed to an insane amount, requiring the enemy speed to be in the 50s to overcome it, thus making Windsweep and the HB/KS combination even more threatening. Of course, such speed may be redundant, and you may want to raise his damage output instead via Odd Atk Wave.