Tetris Ultimate

To unlock this achievement, you will need to achieve 3 badges:
Underdog - Beat the Tetris Self of a player ranked higher than you 50x
Generous - Drop power-ups on opponents 15x in one game of Battle Ultimate
Strategist - T-Spin 5x in one game of Ultra

Underdog
I believe this may correspond to your "belt" level rather than your leaderboard scores. I had a few friends above me on the leaderboards, but even after 100's of rounds trying to play against their Tetris Selves, I could not unlock this badge (my belt level was high from reaching 24 in Endless). So, I resigned myself to deleting my save info and starting again. Once I did that, I was easily able to get this badge unlocked at the low belt stages (Orange/Blue).

Generous
I had this unlocked in my previous save, but needed to get again. I set up a game of Battle Ultimate with 3 players: one a middling Tetris Self (Green belt) and the other an Advanced Tetribot. After 5-7 tries, I was able to unlock this. There is an element of luck to it, but with the bad Tetris Self as a buffer, I had time to activate enough power-ups. 4 players can muddy it too much, and 1 opponent might not be enough unless you can go toe-to-toe with the Master Tetribot (I say this only because other Tetribots are likely to lose to you too quickly in 1-on-1 Battle Ultimate).

Strategist
This will require a bit of practice. I searched for a T-Spin tutorial on YouTube and the first one that came up was extremely helpful. I got this badge within 20 mins of trying it for the first time. Make sure you're playing in the Ultra game type.

I take no credit for the video, but this is the tutorial I referenced for setting up the T-Spin. You should be able to create a pattern for multiples after a few tries.