Dodger2112:How do I get the skull on the 1st mission?  The second I get the 100/100 it's mission complete!

gotta make sure you don't kill them all. There are enough grunts in the mission to where you can have about 5-10 left over after you get 100. Some try to run, try to get those before they do, also some are bombed at the very end, use the warthog smash ability to get them before they are bombed.