Load up the level High Charity. There's a section right after you kill a few brutes that has a Covenant sniper rifle by a jackal at the bottom of the ramp before it ramps back up, then right after this it opens into a much larger area. When you open this door there's a bridge that opens up for you, but don't cross it yet. There are a bunch of flood across the way on a platform, and a bunch of bugs show up to fight them. Take out you're sniper rifle and wait until the bugs have engaged the flood and start taking them out.
You'll need 7 kills in a row, so don't miss. If you do miss, there is another pack of bugs that fly in after the first are down. You can also just reload you're checkpoint. Your checkpoint should be right about when you open that door to go kill the bugs.
Also I wouldn't try killing the flood with your sniper rifle to continue a kill streak, they will typically survive a shot or two.
I found this area much better than the spot in Regret, because when I tried to do it in that level, the platform moving gave me another checkpoint and kind of screwed me over. Here you can take as much time as you want to get this achievement as long as you keep reloading your checkpoint. It's also not too far into the level, there are like just a few fights before you get to this spot, and if you do it on easy I'd say it won't take more than 5 minutes to get this achievement.