Ms. Mediocre

Valkyrie is OK at best. She's really good at minion management, and that's about where her utility stops. Flight of the Valkyrior is too expensive for the setup, why Shieldmaiden isn't a defense event is BAFFLING, Have at Thee! is FINE, and so on.

Here's a bare-bones BASIC deck with the best supports in the game, the Symbiote Suit to bolster her strong attack stat, especially once you have Dragonfang in play, and lots of solid allies.

NOTES:

• Earth's Mightiest Heroes is nice to re-ready Valkyrie for her high attack stat. Ideally, you want to play Blade and keep him in play, solely for this combo.
• Thwarting will come mostly from your allies, as 5/10 of them have thwart stats of 2 or more.
• As stated before, Avengers Mansion, Helicarrier, & Quincarrier are the trifecta of hand efficiency. The 2x "The Power in All of Us" resources will help fund these into play.
• Symbiote Suit isn't necessary, but if the scenario has high health minions or if you want her thwart to become a respectable 2, then have at it.

CONCLUSION:

Nothing special here. It should get the job done and make her somewhat competent. Use her to do what she does best: kill minions. Let the allies do the rest.

EDIT: Shieldmaiden is a defense event, so that's a plus. However, the problem is her "minion slaying" inclination is that she rarely wants her Death-Glow on the villain. This will help on larger health minions, but this could be more focused in a defense deck.