|Taking advantage of his huge HP Pool, Virion can do something that most other archers can only dream of doing; reliably run Panic Ploy. With Valter available, you can strip Panic Ploy from a spare Valter and give it to our Legendary archer, allowing him to invert buffs on every single mage in the game (except for a small handful of 4, running the HP+3 Seal or the extremely uncommon HP+5 A skill). Being as many mages, from speedy to bulky, run #Blade tomes, Virion can position to effectively neuter their improved damage via defense boosts and secure kills on most of them. Not only that, but the same scenario for Panic Ploy applies to every Cavalry-type unit in the game, and all but 2 Flier-type units (Cherche and Beruka just barely skate by with exactly 46 HP like our Legend here). If you're tired of fighting Cavalry teams and want to lead the offense on them, Virion's a great place to start.
Death Blow 3 is important to this build, as it would be for pretty much any Brave build. With 48 effective attack and a Luna proc on Player Phase, he can obliterate just about anybody other than some Armors (Sheena/Hector/Effie/Amelias all survive with 8 or less HP...Hone Atk is welcomed here, but countered by potential opponent Ward Armors). Fury!Sharena, Fury!Ephraim and some #Raven users with Triangle adept are all outside of armors that Virion can blow away. Anything else will simply die. If there's no Luna proc, Virion can still offer quite a bit of damage, but struggles to kill a few bulky mages that hurt him/kill him back (Spring!Camilla, Merric and Nowi in particular), and a couple of outliers just barely survive (Iote's Shield Michalis and Subaki...units that are almost never run anyway). He's surprisingly powerful because the common meta features many units with sub-25 Defense, which is just what Virion lives to kill.
Swordbreaker ensures the KO via quad on bulkier sword users like Draug while still letting him stomp bulky swords that he can't double into the ground, like Ike and Fury!Xander. An alternative would be fine in running Cancel Affinity, but Sword users are far more likely to exist over Gem/TA users. If threats are running a traditional Cavalry team, well you've got a nice counter to them to soften up their approach.
Whenever Virion gets the chance to attack a second time during battle, he can activate Luna. This enemy defense reducing Special allows him to push through more bulky units before they get the chance to counter, letting him cross thresholds against units like Spring!Camilla and Leo. He'll struggle to kill some Armors (Amelia, Hector, Effie), but will come remarkably close, and can utilize any form of damage buff to secure a KO on them, while ignoring any out of combat buffs on the enemy thanks to Panic Ploy.
Reciprocal Aid works pretty well for him, because with his large HP pool, he can restore quite a few units to full HP, with the added benefit of restoring every unit back into a Quick Riposte 3 or Breaker 3 threshold. Alternatives in repositioning can't hurt, but if you're not running any other kind of heal support and feature some other Wait 'n Bait units on your team to counter common threats (Green mages to counter Reinhardt, etc), Virion's the best bowman for the job.
What makes Virion a special case is that his defensive parameters are polarized enough in that even with his shaky speed and embarassing Res, his stats allow him to sponge a melee hit up to 73 Atk, which is a giant number, capable of surviving an 2x Goaded +Atk Death Blow 3 Silver Lance Effie (72 Atk) and 3x Goaded +Atk Hector (67 Atk), a trait shared only by Gordin in the archer pool. With his buffed 26 speed, he can, when necessary, block a route and sponge a hit of melee combat from anyone touting less than 49 attack on a double strike (not unreasonable). His hilariously low Res means that he struggles to deal with just about all mages, but with Panic Ploy in effect, it gives Virion's pathetic 56 Magical bulk some more weight, by dropping mage attacks (and speeds) to more managable values and allowing him to survive a single hit from a handful of mages. To put this into perspective, #Blade Hone Cavalry buffed Fury!Ursula and +Spd Fury!Cecilia and +Atk Fury!Leo all fail to kill Virion if Panicked. With 10 Res, that's incredibly impressive.
If you're concerned about Investment, the game hands you a free Virion which performs almost as well, and with the now-free Valter and previously free Ursula, all of his other skills can come from 3-star units, along with easy access to 3* variants of himself for Nature hunting.