These few of tips will tell you what to do when your energy shields are down and you are facing a large number of Covenant. How to hold them off,how to take cover and when to retreat.
When your shields are down, do not stay standing or you are a sitting duck for enemies.If there is no cover available, move and jump about as much you can to avoid even further damage from plasma weapons.
Grunts can be a nusiance when you're hurt. They will run towards your positon firing, the best thing to do is crouch and take cover in a place they cannot see you easily. Pop out and spray them quickly with your weapon then duck again. Or you can come up, throw a grenade into their group and duck again. This will disorientate and seperate them.
Elites are tougher, retreat backwards but stay facing them, now and then give them a short burst of gunfire to hold them at bay, another tatic is the hide, release, charge as I call it. If you are on level with twisting, maze like passage ways, for example Pillar of Autumn use this tatic. As the Elite follows you, hide around a corner and as he gets close let loose a grenade, pull back, then after it detonates, attack the Elite.
When taking heavy fire while in cover it is best to switch to your most powerful weapon and spray the enemies with it, duck reload and do it again.
If there are several enemies(Elites, Grunts and Jackals) in one big group attacking you when your shields are down, just throw all your grenades into the crowd.