Make sure to tell the other characters to support you when attacking a frag.
When in combat, tell your teammates to fire at will, move around a lot, and throw grenades when there are a lot of aliens grouped together.
Always pair one guy with one girl when playing regularly.
To clear out a lot of enemies, have Tex and Brutus go in and do as much damage as possible. Then, retreat back to Flint and Hawk. Also have Flint give you sniper support and have Hawk cover Flint. It also helps to send Hawk in invisible to scout and kill as much as possible.
When going into any hallway or cave, keep Flint back, but able to snipe clearly into the hall or cave. Put Brutus on one side and Tex on the other outside of the hallway. Have Hawk explore, and if needed lead the enemies out. They will be blown away from front, left and right and from Hawk.