Halo 3 Catch Skull Help?

[nuttrong]

Now I'm getting these skulls and so far I have the Iron, Black Eye, Tough Luck, Blind and Grunt Birthday Party skulls. Now from what Ive seen on the net it's on a large cylinder type building next to an anti-air tank.

Here's my problem, it's never there. Here's what Ive tried/been doing:
-Co-op
-Single Player (Tried single once and used a sniper to check if it's there from that building all the marines are cornered in when you enter)
-Normal Difficulty
-Hard Difficulty
-Not touching the tanks at all (most times I try)
-Shooting everyone besides the tanks (sometimes humans after I get annoyed about the skull)
-Having all my skulls (I have so far) off
-Putting all the skulls I have on
-Turning only one skull on (Blind Skull)
-Team scoring on
-Scoring off
-Jumping on master chief as the arbiter to get up (I use a warthog I find flipped in that area and drive it behind the building at the cliff then player one jumps on the warthog and player 2 gets up)
-Snipping the Wrath passengers (about once or twice)

Those are all the ways I have done or what I can remember doing.

What am I doing wrong? Why is it not there? Is there a certain order you must collect the skulls? I'm stumped.