This can be done on any difficulty, so it shouldn't be a hard achievement. Just the opposite, if you are playing on easy, you can pretty much just run through everything. Just be careful with the rocket flood.
This is a level that you play as the Arbiter, and the main enemies are flood and sentinels. The Carabin should be a good enough weapon for most of them, and if you manage to get a laser from one of the sentinels, it can also be a good choice.
In the end of the level, sometimes a flood can get stuck in a corner or in the scenario. If the level doesn't end, just look for it. All flood must be eliminated.