in halo 3 how do you get the catch skull...?

[Red Sox Ted Williams 9]

i know its on top of the silo but i go up there and its not there and i didnt kill the purple wraith only the AA wraith

 

[sublittle_tank91]

TO get the Catch skull, you have to allow the AA-Wraith and the Normal-Wraith to survive. You can still kill the Ghost though. This can be easier accomplished if you play in Co-op where one player disable the Normal-Wraith with a charged Plasma rifle and the other one just getting the skull...

Also ensure that the Marines do not kill the AA-Wraith.

For more information,
>http://halo.wikia.com/wiki/Halo_3_Skulls