The easiest way is to simply board one and destroy it. But the Wraith weak spot is on the back.
but how do i kill one in hola ce in assult on the control room the first one by the overtunred warthog?
To kill the first Wraith in "Assault on the Control Room", just simply grab the rocket launcher behind the overturned Warthog, sneak up on the Wraith, and fire when in range. Tip: Make sure to avoid the enemy infantry and protect your Marines.
there is a rocket launcher, that gost must be distracting me, could i get the sniper rifle and shoot the elite out of it?
No. It would take a heck long time for that.
Or you could just get on top of the hatch and fire a gun. The shotgun or assault rifle works best. Just make sure the mortar doesn't hit you. The rocket launcher is usually for head-on approaches, but in everything you need a rocket launcher for there's actually a much more complicated way, but often requires less ammo and time.
Or still you could get the warthog and use it's turret.