16 tries later, I finally beat it. Good call on the Critical Boost, that finally made the difference for me. For my winning battle I had that, +2 All, and 60% Match Damage. I was able to make 3 corner 5 matches, while Thanos made two critical matches and three cascade pink matches. The only reason I won is because he matched away his green countdown tile immediately after placing it. I ended the fight with 223 hit points remaining. Brutal. So victory is possible, but Thanos is amazingly fortunate with his tile drops and placement, considering he and KK both need pink and green.
Got a blue War Machine for my trouble.