Re: battle.
Rocket is a front lines type, so he'll be with Groot likely zipping around on his jet pack and blasting from the sky.
Loki will be with Ben, doin' his magic thing and being good about not straight-up bludgeoning anyone to death with his scepter. >_>
Barry is putting his speed to work and making sure that civilians are all out of harm's way. He's more protector than fighter, so he'll be running around and yoinking enemy fire from the air as well.
Lance is his team's cover since he's a long-range shooter. He's also probably forfeiting his own cover to do so, so we'll see how that shakes out.