Libra

"...but in the everlasting darkness there shone a light brighter than the sun itself. It flashed with unending vigor, reflecting off of any who stood alongside it. The more he bled, the stronger the light grew and renewed his strength with every swing. An unstoppable crusader imbued with great power. Libra: Herald of Purity."

(One half of my Yin/Yang build with Tharja).This build can work with any refinement of Ardent Service+, although I personally recommend (+atk) for more flat damage. It also works regardless of IVs as long as you have a HP bane. For example, my Libra has a res boon and he can still take on just about anything you throw at him. This is mostly due to the fact that as soon as he drops below 80% health he gains an additional +14 atk as well as +7 def/res and at 75% health his special does +10 damage. Ardent Service+, Joint Hone Spd and Rally Up Atk+ all let him support his entire team both offensively and defensively as well as buffing his own atk and spd. Sol can obviously be replaced with a more damaging special, but between the +18 atk and the +10 damage when he's missing enough health, Libra will have little trouble taking down almost anything in his way and will more likely than not find himself recovering 10-20 health with every use of Sol. It's also worth noting that while Wo Gún+ (+atk) will allow you to deal +20 damage on special triggers, Ardent Service+ (+atk) is able to raise Libra's atk to a solid 55 before dragonflowers and merges (58 if he has an Atk boon).

Seal: Heavy Blade 3

An untested experiment build for longer game modes.

Heavy Blade works well with Libras high attack to cut down some of the charge time of Aether, which combined with Wo Gun+ effect deals massive damage and heals him up nicely. A +Atk nature and +Atk Weapon Refine both increase the damage/recovery from Aether and increase the reach of Heavy Blade.

Distant Counter and Quick Riposte are both taken to cooldown Aether faster - with Heavy Blade, it should be ready to go fairly often. Unleash it on fresh enemies where you can to keep his HP within Quick Riposte range. With Aether restoring his HP fairly regularly, a -Spd nature can be taken to let him be doubled for faster charge.

Atk Smoke 3 is kept for survivability, since he is going to be trying to tank.
Pivot is taken for maneuverability, but it is not sticky.

(+10 Merge and Dragon Flowers Included)

This build utilizes Libra as the War Monk that he is in Awakening in which he will function as both a passive healer and an offensive unit.

Initially, I wanted a budget support unit for AA, and I thought Libra would be a great candidate due to his great mixed defenses. Also, I wanted to fodder Ardent Service, since I never used Groom Marth. So Libra, being an axe user, seemed like a great choice. But it turned out that this build is extremely effective -- so much so that I'm considering +10ing him.

First of all, Ardent Service and Hone Spd combine to give adjacent allies +4/+4 attack and speed, making Libra a great budget buffer. He becomes an excellent teammate for offensive sword units like Ayra, Karla, Hana, or Athena, and can even buff blade tomes in a pinch.

Next, Libra's IVs (+spd/-atk) are excellently suited to this build. His speed -- 38 with a speed refine -- is enough to ensure he often won't get doubled, thus contributing to his defenses. Also, Spd Smoke in the C-slot helps him get doubles of his own, while also helping his teammates get doubles.

Triangle Adept and Guard combine to make Libra incredibly tanky against blues. He might be the best blue counter in my barracks. Literally no blue unit (besides Ophelia, I guess) is a threat against him. Guard protects him from Fighter skills and most specials, and Triangle Adept reduces damage from Blues, as well as giving him more damage of his own. This makes his -Atk IV very ideal, since having -Atk preserves his excellent 28/29 defenses and his great HP of 45. Also, those 28/29 are further augmented by TA.

All in all, this a wonderful build, and extremely effective at doing what it's supposed to do -- tank all blues and provide excellent ally support. You can safely leave him in range of Reinhardt, Legendary Tiki, etc., whlie buffing your offensive units with Ardent Service + Hone + Spd Smoke.

His weaknesses are obviously Red units and some other Green units, but since he's part of a team, this shouldn't really matter. If his part of a competently arranged team, he should have teammates ready to take care of the things he can't handle.

So if you're looking to give something different a try, I recommend this Libra build. He'll be a great addition to your AA team or your Arena core if you choose to +10 him.

One half of my death dealing duo, Libra is the wall that stands between Virion and instant death. By no means the most tanky unit in the game, he serves well as a fierce barrier that takes wonderful advantage of Virion’s softening up to finish what he started. Fury/Renewal keeps Libra going, reposition pulls Virion out of trouble, and threaten defense makes sure whatever Libra’s after isn’t too heavily fortified.

Laugh as you will, but by Naga’s grace, there’s not much of anything that gets past Libra.