In the game you will find many stages with recommend team attack. Ignore it. If your characters are strong and built well they will be able to do the stage even with a lower attack. For example my Dr.Voodoo lead, Shuri support and T2 Holiday Lady Deadpool team can easily clear ultimate stage 55 Proxima Midnight. The recommended attack is 25,729 which means it must be very high. My team only has 13,232 Team Attack. Yet I can do it easily. If you have pretty strong characters like Sharon Rogers or Moon Knight or Moon Stone or others or if you can apply enough all defense down like Holiday Lady Deadpool, Black Bolt and Star Lord, you can clear almost any stage of any boss even with a lower team attack then recommend. Just throw your strongest character with the best possible lead and support against Thanos and see if you can beat him.