Test deck. Been using this in casuals on Conquest mode and it seems to do decently. The idea is pretty simple. Flood your lanes quickly using a combo of 1 Costs, Brood and Mysterio, then use Valkyrie and the Ongoings from Dazzler, Patriot and Antman to steal lanes. This deck can dump a deceptive number of cards down on the table, so it is very easy for Turn 6 to be a massive power swing. Some notes as always:
It is usually better to hold most of your cards until later turns such as 4+. The main exception is Turn 3, where once you see all 3 locations and your 6 card hand, you can figure out how you want to stack your power. Generally speaking, Turn 3 is best putting down one of Brood or Dazzler. But in general, you want to lose priority anyway because it prevents the risk of a card getting Shang'd or Valk going first.
Bast is pretty much a pure power gain card anytime you use it. Obviously, the main candidate you want to hit is Mysterio, but hitting Brood, Hood and Wasp are all very good. I swapped Sentry out due to his incompatibility, but 10 power is nothing to scoff at, and his downside can be easily mitigated. YMMV.
Tech card choices are tough. Cosmo basically kills Valk so I don't recommend using it. Shadow King will neuter the Bast boost, so similar, you have to be careful. Shang doesn't fit on the curve so while he's okay, I don't think he fits in great. The only card that doesn't have any issues is Rogue. Rogue is also good because one of the counters against this deck is Luke as he will prevent Valk's power drop. You can play Rogue to steal it, and then still buff her with Surfer. As always, tech choices come and go depending on what the flavor of the month is, but this is probably the most "standard" build.
As for your subs - Ghost is only really recommended if you have issues dealing with priority. Otherwise, you can completely skip him since he doesn't add much else to the deck. I like him though since he's added insurance in case your opponent realizes what you're doing and tries to lose priority. Bishop is a good sub because of all the 1 costs you play + Mysterio gives him a+3 activation bonus. Also breaks parity after Valk. A good option to replace Dazzler or Surfer if you don't have either. Squirrel Girl is another lane flooder and has Valk, Dazzler and Patriot synergy BUT makes you even more vulnerable to Killmonger. Card Choices:
WASP - Pretty self explanatory. A 0 Cost card, that can be buffed by Valk, Patriot and Bast. Plugs holes for Dazzler. Very versatile overall.
HOOD - Hitting Hood with Bast turns it from a 1, -3 to a 1-3, so a massive 6 point boost + you get a Demon to play. Hood himself can also be used as a sacrifice, or a warm body for Valkyrie. The Demon has Patriot synergy and can be played on Turn 6 for a 1-8.
ANTMAN - A 1-cost Ongoing that breaks parity after Valkyrie. Not only that, but you're looking to fill lanes for Dazzler anyway, so Antman is a pretty natural fit.
BAST - Pretty much anytime you use it, you'll get value. The later the better, though don't be too late about it.
GOOSE - Guards your Patriot from Enchantress and makes Rogue a 50/50 if the opponent wants to chance it. Also stops large drops from being played here, which is good because it also makes it impossible to play Valkyrie here. All your other cards won't have this issue.
MYSTERIO - Bast value is a 2-9, has Patriot, Antman, Dazzler and Valkyrie Synergy. Also, the Illusions if not Bast show up as 0 power so they don't skew priority at all.
PATRIOT - Self explanatory also. About the main thing I'd want is like IW or Goose to Guard Patriot, but it's hard to fit him into this deck...maybe over Squirrel Girl
BROOD - Also self explanatory. Playing Surfer, Patriot, Bast and NOT adding Brood is insane since he presents a 3-21 or something nutty. Great establisher with lots of versatility. Works as a last minute lane dump too if you need it.
DAZZLER - Is basically a 3-8 in most games here, so great value for cost.
ROGUE - As mentioned above, is the preferred tech card to carry. Kills an enemy Ongoing that can spoil Valk, has Surfer synergy, etc.
SURFER - Works with all the 3 cost cards here in case Plan A doesn't work. You can even add more if you want, but then it becomes more Surfer build then a Bast Valk Build.
VALKYRIE - The big gun, so to speak. Valk needs set up but is a tide turner if played. You can pretty much steal a lane with her if you're set up right. Be especially careful on Turn 5 to avoid Priority. I originally had Yellowjacket here so you can play Wasp, Yellowjacket, Antman, Valkyrie to steal a lane guaranteed on 6. However, what I found in practice is that this rarely lines up, so I removed Yellowjacket as a result. Still, having Antman, Wasp, Valkyrie works as long as one other card is in the lane and THAT should be easy to accomplish with Squirrel Girl or Mysterio.
SQUIRREL GIRL - With Patriot and Valkyrie, those 1-1 Squirrels can get up to 5 power...pretty good! Also fills lanes for Dazzler, Antman, etc. Makes you more vulnerable to Killmonger, so be aware.
GHOST - You want to not have priority like most of the game so Ghost helps with that. Best played on 4 in case of sneaky Killmongers.
BISHOP - As noted above, breaks parity and will gain plenty of power due to all the low cost cards in the deck.
SHADOW KING - A choice 3 cost tech card that doesn't kill Valkyrie but is anti-synergistic with Bast. However, it resets big boosts on the enemy cards, which if you play it correctly can really spoil someone's day (BOUNCE). Works with Ghost and Invisible Woman to guarantee reveal last.
SENTRY - Valkyrie kills the Void. 10 power is respectable and stands out since most of the deck is low powered to start.
Honorable mentions to Armor and Invisible Woman since both will help protect some of your more vulnerable cards.