Before you start the engagement, think in terms of bottlenecks and suppression. Make sure you know where the ammo crates, health packs, energy cells and suit recharge points are. In other words, do some reconaissance before you start fighting. Force the enemies to fight you on your terms. Roofs are good, but make sure you have plenty of space to back away from the inevitable grenade spam. Putting yourself at the top of a flight of stairs is also useful, as it forces the NPCs to come up at most two abreast, and when you fire at them, they'll try to use the stairs for cover. You can then pop forward, shoot and duck back, forcing the NPCs to attempt to maintain their cover. If they run away, they often run towards their mates, so don't get too greedy and chase them - you will lose your situational advantage and may be swarmed.
Remember, you can switch suits in your ship/SRV, so open with the Dominator suit and switch to the Maverick after you've killed most or all of the scavs (some might be inside a building, perhaps).
If you're playing with a controller, reconsider keyboard and mouse. I don't think controllers have aim assist, which makes landing your shots a lot harder. Make sure to aim for the head once the shields are down - there's a headshot damage modifier, and the time to kill is so high that you get plenty of time to get the shots in. In general, the plasma weapons are pretty hard to use. The Executioner (marksman rifle) suffers from very slow projectile speed, so while it's powerful, it's a difficult weapon to use. It rewards skill and experience with how the bad guys move. Use the Takada Aphelion laser rifle instead - it's very effective against shields, and is effective within the same range as the Executioner. With the Dominator, you get two primary weapons slots, so equip the Karma AR-50 kinetic assault rifle - it has basically no recoil, a decent magazine size and high shot speed, which enables more accurate fire.
Alternatively, just go in with your SRV - it's actually pretty tanky. Switch between 4 pips to shields (when you're taking fire) and 4 pips to weapons (when you're doing the shooting), target the NPCs (who will stupidly try and swarm you), then blow them away with the plasma repeater. For bonus fun points, you can run NPCs over. The screams are hilarious.