I have passed halo 2 on normal and heroic, but I don't know how to get..

You can only get the skulls on certain levels (I don't know the actual levels) and you have to do it on LEDGENDARY.

You can only find skulls on Legendary. I don't know where all of them are, but I can tell you where you can find the Grunt Birthday Skull (when you shoot a grunt in the head, they blow up like a plasma gernade).

On the Arbiter level, when you see the Heretic Leader get in his Banshee for the first time and you must escort a Phantom to where the Heretic Leader landed, get in a Banshee and fly down. Eventually, you'll see three cylinder-like things.

Fly to the middle one and turn right. Under a support beam, a bunch of Heretic Grunts will be dancing around a skull and they will not shoot back at you.

Hope this helps.

You can find one skull on the Outskirts on any difficulty. This skull is the blind mode skull. At the crash site move forward through the hall and kill the grunt to your left. After that look up and there will be a light.

Jump on it,it is small so don't fall. Turn left and jump on top of the building. Look left and walk down the ally and there's the skull. (You can also skip half the level this way)

Just so you know, there are skulls on practically every level but you can only use them on legendary.

Except for the Blind skull, you can use it on any dificulty.

You get them on legendary

You can only get all the skulls but one on ledgendary the one skull you can get on any difficulty is the blind skull in outskirts.