They don’t even know why they’re fighting you. There’s no fun to killing Prometheans, and there is no fun in being killed by a Promethean. You have to empty entire clips into them, just to kill one. The only difference is that they are tough to kill.
The Prometheans are by far the worst enemies in the entire Halo series. When an Elite’s shields were low, they chased you, same with berserking Brutes. They ducked and dodged, used the vehicles, torrents, and the environment to gain the upper hand. Most importantly, the AI for Halo 1 – 3 were much, much smarter. I can’t count the number of times I laughed out loud when an Elite killed me, stood over my body, continuously shot my dead body, and talked smack. It was amazing to hear the Brute leader say, “Hmmm, go see what’s taking them so long,” or an Elite says, “For the great journey!” or “Get up, so I can kill you again.” These quotes not only elevated the stakes, but they gave you insight into your enemy’s military ranking, tribal interworkings, and motivations. They uttered the classic “Wort, wort, wort”! That utterance was a source of endless memes today. The Elites in Halo CE were just as lively even though they didn’t speak English. When you got the upper hand on a Covenant unit, the Grunts run away in panic, uttering words of fear while the Brutes would go “berserk” when they notice you killed one of their pack. They don’t verbalize clues as to their motives, express enthusiasm in fighting you or fear when you have them outmatched.
They express nothing when you fight them.
They hop or transport (cheap sci-fi strikes again) here and there and run around like moving targets, not beings with life. The Prometheans are nothing more than bullet sponges. This applies mostly to the Prometheans but can be applied to the Covenant as well.