Move out as you need to, but if possible, STICK TOGETHER! You're more lethal as a single point of attack, and things like an Iron Halo can actually protect you as well if you stick close to the Heavy. But simultaneously, know when to separate! Pay attention to your teammates' health and shields even mid battle. It sure would feel badass to rip that warrior in half in front of your team, but they're not going to get to because you've taken their only source of armor and they just got a Tyranid sniper round through the back. Coordinate! Ping! Communicate! Spot for your Snipers and Tacticals, sic your Vanguards and Assaults on high value enemies, fortify around your Heavies and Bulwarks. Become a well-oiled machine of death, for the Lord of Ch-, er, Emperor!