3. Soldiers( Halo 5): I realize how hard these guys are on any difficulty. You don't even need to have the angry skull on to make them shoot faster, they pretty much already have it. they just never die.
2. Jackal snipers or anything on halo 2 legendary: a lot of people hate these guys. They are ones hitters. You think u are all tough on legendary with big armor but actually you are the second most weakest thing in the game. The only thing that's weaker than you are flood infection forms.
1! Skirmishers(Halo Reach): I ABSOLUTELY HATE THESE THINGS! They are so annoying on all difficulties because they will evade everything, u can't land a hit on them. Also, even if u do manage to land hits, they take so many to kill. It takes like 3 or 4 melee attacks to kill one.
Dishonorable Mentions:
Hunters(Halo 4 and 5): you can't use the old evade then shoot the back strategy anymore. They can hit u either way pretty much now.
Elites(Halo 4): hate them. They will mess u up big time. They shoot a whole lot aggressively and take a while to kill high ranking ones.
Jackals(Halo 5): they were so weak back then, until an update came in which they will almost one hit u. It's not the jackal snipers, it's just regular ones that takes around 5 hits to kill u on heroic or legendary.
Prometheans in general: play halo 4 and/or 5 and you'll see what I mean