Steven Universe: Save the Light

To perform a fusion or a team attack you need to have the friendship gauge filled up, for that you compliment one character after they perform a perfect block( you can choose whoever you want to give the compliment), use a friendly skill like heal and protect( for some reason not all 'boost' abilities do that) or use the 'gift' item, this last one also works out of battle. Now there is a total of 7 playable characters and they all have these attacks or fusions with each other so that gives us 21 combinations. In game there is no record of which ones you have already done so be sure to track it yourself, here the 21 that you need to do:

-Steven-Connie: Fuse to form Stevonnie.
-Steven-Greg: They sing together healing all the party over the hp limit.
-Steven-Garnet: Steven bounces in a bubble damaging enemies.
-Steven-Amethyst: Fuse to form Smoky Quartz.
-Steven-Pearl: Pearl receives a powerful buff.
-Steven-Peridot: They sit together giving stars to the team for a while.
-Connie-Greg: Connie receives a powerful buff.
-Connie-Garnet: Both execute a combo on an enemy.
-Connie-Amethyst; After a small charging time both receive 4 stacks from the 'study' ability.
-Connie-Pearl: Both execute a combo on an enemy.
-Connie-Peridot: Peritod create 4 mines and throw them at Connie, each one she manages to bounce back is planted on the field.
-Greg-Garnet: Both play their instruments dealing damage in waves.
-Greg-Amethyst: They create a giant sound wave that stuns and damage all the enemies.
-Greg-Pearl: Pearl receives a powerful buff.
-Greg-Peritod: They place Greg's van on the field then it rolls over the enemies.
-Garnet-Amethyst: Fuse to form Sugalite.
-Garnet-Pearl: Fuse to form Sardonix.
-Garnet-Peritod: Peridot bounces in a pod dealing damage.
-Amethyst-Pearl: Fuse to form Opal.
-Amethyst-Peridot: Create a special turret that stuns then debuff the enemies stats.
-Pearl-Peridot: Create a powerful canon.

Keep in mind you only need to form the fusions, not to use all their abilities and that if you make a team attack or a fusion and for any reason you need to leave or quit the game(like when it bugs), it will not count.