Algodoo

Credits to retxabb for the original "Super Smash Bros. Ultimate"

Use images or text on squares to portray a character. Drag them onto to the ground. Press play and watch the battle! (Cell and Frieza are examples)

I recommend you create a square that's "0.25" meters and put an image on it, then click "Generate geometry from texture" and adjust the size to fit the character/image size.

Also you can increase the density of a character and they are more likely to win the heavier their density is.