There are a few ways to get this that are pretty easy.
1) Set difficulty to easy, defend until a unite ability is at 100%. A character’s unite ability increases 10% every turn. Being hyper provides a temporary 20% boost. You could kill all but one monster as well. This can be done as soon as you unlock unite abilities.
2) Slight spoiler for later in the game.
*** Spoiler - click to reveal ***Imogen gets a passive that makes all her unite abilities 100% while hyper and another passive that starts her with hyper. So, activate both of those
and then get into a battle or choose battle from the main menu.
3) During the game, you can find a useable item that increases unite percentages. Incorporate this into process 1 above to speed it up.