1. I don't believe that Valkyrie is the problem. Since zombies play first the plant player is guaranteed a turn to deal with the Valk, at the very least they can throw something in the way to take the damage. The real problem is teleport. This bypasses the plants ability to react. Have you played Valk in other decks besides Brainstorm? It's just not as powerful, it's decent sure, even good, but not ridiculous. I'd suggest teleport either have it's cost raised, the card draw removed, or limit it to zombies of a certain power level or lower.
2. Power Flower, it's a good card, no doubt. It's hardly unbeatable though. 5 Health is hard to kill instantly, sure. It's only 3 damage though, and 1 heal per turn by itself. It dodges much of the zombies available removal, but not all of it. Locust Swarm always works, B-Flat has a chance to work, etc. As someone else mentioned every Gargantuar kills it too. Power Flower is a 5 cost card with only 3 power, how many cards cost that much and have so little power? Usually only ones with very powerful abilities.
3. Repeat Moss - It's a solid card, but it's a 2/3 that costs 4 sun. That means it can't be effectively played before turn 5 and even then only 1 trick can be played that turn. That results in 6 damage at best, with Combustible's superpower, Then if the lane started out clear the attack phase attack does another 6, totalling 12. It's not an OTK, it's subject to shields, Zombies can be moved in front of it during tricks, etc. It's a solid card but not unstoppable.