This could be setup as a new grenade or weapon shot (I'm thinking like a rocket) or ability. Imagine a wall of flames on the ground (similar to the Hunter Nightstriker Voidwall grenade) that when your fireteam shoots through the flames or lightning, it changes the attack type of the weapon to the element of the flames. Lets say you love a weapon but need to switch it out for a different one that does a different elemental damage type because you need to bring shields down. Instead, someone on your team throws this new grenade or rocket shot or special between you and the enemy, your team all lines up behind the wall of flames/lightning and shoots through it and all bullets that pass through are changed to the element of the wall. Only lasts a few seconds but gives you the benefit of taking down a shield without having to change your weapons.