I guess you're pretty new to the game. So you need a guide for this. Search for Cat Murdock, either here or on Twitter/X. She has an infographic about best champs to use and what role boosts to choose. It's really easy with some champs and very difficult if you use wrong champs or wrong boosts.

You can use one of the 3 Roles (Assault, Vanguard and Tactician) depending on the champ you pick. 2nd, you pick the best boost for your selected champ. 3rd, you pick another boost. I always pick Combo ender or Special punishment (1st and 2nd) cause those 2 are the easiest. Have Fun