Long-distance grenade

When you've cleared the first enemy area and dispatched the arriving reinforcements, there's a distant Grunt in a Shade, along with a pacing Jackal. See if you can take them out with a long-distance plasma grenade (ideally before they've even seen you). You'll need to time it so the Jackal is near the Shade when the grenade lands. Once you've found a good way of deciding the right throwing angle, you should be able to do it every time. Use the sniper scope to watch the Shade and Grunt fly off into the void.