Strongest deck I've made so far was a flow deck with that earth card that adds another copy to your deck if you have flow. I upgraded it to add vulnerable and half mana cost so if it was the last card in my deck and I had flow I could spam it infinitely. I also had an empower card that increased the damage of the other card that was slotted so it was doing quite a bit of damage.
I still couldn't beat it but I made it to the last boss before the final area I think.