One Piece: Burning Blood

If you, like myself, are having problems getting this achievement because you're new to fighting games, there is a really easy way to obtain it along with any of the other "Unity" achievements.

If you play through the main campaign as Luffy, you will eventually reach the 6th stage called "Battle in Oris Plaza!" where you play as Luffy and Ivankov against a team of Pacifictas. This stage is a bit different from the others as it serves as an introductory level on how to use these special team based unity moves. The battle will periodically pause and show button prompts explaining how to execute each type of unity move.
However, the reason I recommend this particular level is due to the advantages. During this fight you will have no time limit and your special move meter will be constantly full meaning that you can spam these special move combos without having to worry about refilling the bar or running out of time.

For this particular achievement you have to swap characters (LT/RT) just after you blow an enemy backwards with a powerful attack. You can repeat this battle as many times as needed. The count is cumulative and doesn't need to be done in a single battle. In fact, you may already have some progress towards this from previous battles as well from accidental button mashing.
Have fun! 😁