From Halopedia, the Halo wiki
Being the only Covenant individual found on this level, Final Grunt is found cowering on the edge of a cliff right before the player goes down the ramp to leap into UNSC Forward Unto Dawn's hangar. To find this Grunt, turn the vehicle slowly to the right, and you can spot it at the end of the narrow pathway. To hear its dialogue, make sure your Warthog's chaingun out of range, as in single player the Arbiter will kill him, preventing you from hearing the quote. If the final Grunt is killed while saying his dialogue, his dead body will carry on speaking. If he is knocked off the edge, however; the dialogue will stop. Discovering the final Grunt may leave the player unable to complete the end of the level as the platforms would have broken apart, thus rendering no possible route available to reach Forward Unto Dawn.