Extra Nightly Cow
That skull is a pain in the a**. I recommend playing on co-op and have arby just stand in the upper level so you can kill him and grab his plasmas over and over. Grenade jumping is easier with plasmas, imo.
Just keep at it and you'll get it eventually. I heard some people used a rocket launcher to knock it down, but I've never tried it.
Good luck!
Thanks! I also heard if you have a chopper, and you go around far enough, you can drive up the rocks, park the chopper on the slant and jump on the chopper to help you get up. I have never tried it though. But yeah, thanks!