A bunch of more ideas.
Enemies always seek for cover. It should make them harder to kill and you need to use the environment for your own favor. For example:
-You are in a flat ground, there is an Elite with a sword 50 away from you.
-You see him, he see you.
-You pull the trigger, your weapon (now use the BR) shots 3 rounds.
-He seen the muzzle flash and hears it’s sound, he ducks away (Halo Combat Evolved style, because it’s one of the best evade ever in Halo). Your weapon isn’t hit-scan.
-There is a cover at his right, 15 meters away.
-He starts running for it.
-You shot 2 more bursts, 4 shot hit form 6, thanks to his evades.
-He is made it to the cover.
-You throw a grenade.
-He can’t see what you doing, since there his vision is blocked.
-He recognize the grenade, he can’t duck away but can go to the edge of the cover.
-Grenade blown, his shields are of, he is at the peak of his cover.
-You can see him, aim at his head.
-He knows, a headshot be fatal, so he uses his hands to cover his head.
-You fire with your weapon.
-1 bullet missed the target (evade mechanism), 2 bullet landed. One on his left arm, one on his right.
-His Sword turns off, since the bullet damaged his hand but he made it to the cover.
-He then throw a plasma nade, in an arch near you, you need to duck away.
-There is another cover, 20 meters away from him, he tries to run for it.
-His shields are now full, he knows where to throw a grenade to suppress you, he do an another thrown.
-As the grenade landed, he starts to run.
-You fire again, 3 bursts (7 hit from 9 projectile).
-He now use a Halo: Reach style evade to made it to the cover.
-Now, only 15 meters between you and him.
-He now throws a grenade again, you need to get out of it’s blast radius.
-He now put everything into one card, now he charging at you.
-You see him coming, you start to fire with your BR, 4 bursts (all landed, he dose not dodge because every seconds count).
-Your magazine is now empty, he is no shield.
-You need to pull of your magnum.
-Your bullet hits his head, he dies.
-There are only 3 meters between you and your enemy.
It’s sounds silly right now, but the basic of this is the mechanism is when the AI is seeks for cover, and dose not make idiotic steps, like charging at you from miles away.
The second is when they are making a trap for you.
For example:
-You are in a hill, there are Forerunner ruins on the lower ground, approximately 70 meters away.
-You see a pack of grunts, 6 of them, one in a plasma turret, one with a Fuel Rod, and 4 more with Plasma Pistols and Needlers.
-Seems to be and easy target but you’re not fool enough to charge at them, you starts to scout the area for more enemies.
-You now see an Elite, Major class with a Storm Rifle. It’s believable, 6 grunts for 1 Elite. You believe they don’t seen you since they are not attacking you.
-You scope in and pick the Fuel Rod guy first, then aim at the turret guy.
-Your shields are suddenly gone, you got a hit from your right, 2 more grunts with Plasma Pistols.
-“How the hell did these guys came up there?” you asks yourself, and find yourself confused.
-You picked up the two grunts quickly, but the other 4 guys at the ruins are gone.
-Then you got hit again from the ruins.
-There is a marksman jackal there, you can’t see him before, because he noticed you while you’re scouting the area and hide at one of the blocks.
-You need to get to cover, you’re too vulnerable now.
-The pack of the grunts are now starts storms you, they are coming up the hill.
-The Elite also shows up, he us suppressing you with the plasma turret.
-Your shields are up, you get up from the cover and kill the 4 grunts.
-The elite leaves the cannon, while the jackal shotting you to keep covering fire for the Elite.
-The Elite runs away, you can’t really see him because of the jackal.
-The jackal then empties his Carbine’s magazine, he ducks for cover.
-Your ammo count is low, you pick up one of the Plasma Pistols for backup.
-You now start to get revenge and go down the hill at the same way where the two grunts came up.
-You meet the Elite there.
-He is really angry, he tries to beat you up, 5 meters between you and him.
-You can kill him thanks to the plasma pistol, you then pick up his storm rifle.
-The jackal seeks the higher ground for you, he can’t see where you went, he is panicking because he can’t see his leader.
-You got a good angle at the jackal, you starts to suppress him.
-He ducks for cover again, you storms him.
-Your rifle overheats, the jackal is on the ground.
-You beat him up.
-Finally, you killed 8 grunts, 1 Elite and a jackal but you lost most of your ammo and nearly died 2 times. Are you really ready for a second encounter?
or a shorter version:
-You can see a ranger elite at the end of a tunnel.
-The tunnel is dark and has a lot of cover.
-You are in an advantage, he can’t see you.
-You pull of your plasma pistol, noob combo him.
-You now starts to walk at the tunnel, you’re a good boy.
-You pass his dead corpse, then something catches your eyes at your radar.
-Three blue dots, approximately 5 meters from you, one at your left, one at your right and one at a random cover.
-Three zealots with swords and active camo.
-Now you can start to pray for your life.
Third, join forces.
Same example again, but the Elite runs away, he didn’t attacks you. He knows he at a disadvantage since his units are killed by you. The jackal still tries to slow you down, to cover the elite’s escape. Then the Elite could came back with backup, or he joins to the others, and waiting for you at the next encounter. Your ammo count is low again, you need to scavenge while they are form up and waiting for your approach.