what game are you playing? I haven't sen Thanos in Pokemon Go, for example...please provide more detail on what game you are currently asking this question for.
It is hard to answer when we don't have enough information on the game, I did a thorough search and could not find such a game where pokemon goes against Thanos.
Please provide us with more details and we can help you out!
Unfortunately, there is no one Pokemon character that can defeat Thanos. The best strategy is to use a combination of Pokemon characters with different types of attacks to overwhelm and defeat Thanos. Some good strategies include using Pikachu's electric attacks, Charizard's fire attacks, and Blastoise's water attacks. Other strong Pokemon such as Dragonite, Gyarados, and Machamp could also be useful.
One way to defeat Thanos is to use a powerful attack that nobody else can join in with. Another way is to use a powerful attack on him that is too slow or ineffective.