A double melee attack is a quick way to kill an enemy on Halo. However on Halo: Combat Evolved it’s a little bit different… You don’t exactly lock on to your opponent when performing a melee attack like in other Halo games. And not just that, it’s not possible at all unless you’re carrying at least 1 grenade.
Technically there is no such thing as a double melee kill in Halo: Combat Evolved because it takes 3 melee attacks to kill an enemy (unless you are running towards your target when both melees land). Keep in mind that different guns have different hitting distances and melee speeds so you will have to time your double melee attack differently depending on the weapon you have at hand.
If you have a spare grenade you can do a double melee attack. What you need to do is melee an opponent, quickly throw a grenade, then melee them again. So the button combo is just melee, grenade, melee. Basically what’s happening here is the animations are cancelling each other out, which allows you to do 2 melee attacks in quick succession at the cost of a grenade.
Whenever you do this, be sure to get away from the blast caused by the grenade so you dont take any damage yourself. You will find that a double melee attack isn’t quite enough to kill an enemy in Halo: Combat Evolved (if your opponent is at full health and shields), but there’s a good chance your enemy will die from the blast radius of the grenade you throw.
Just keep in mind that grenades only take about half a second to explode once they fully settle. Plasma grenades on the other hand have a 3 second detonation time once they fully settle, and are more of a safe bet to do a double melee attack with because you have more time to get away from the blast.
A little tip is to shoot your opponent before you do a double melee attack. Just enough to get their shield down a bit. This way you can be sure that the double melee will kill them, because sometimes they can get away from the grenade blast (especially if it’s a plasma grenade).