On the level "The Ark" its on the overhanging platform that goes in the direction you have to to secure the "Dawn's landing site where you get the tanks. The platform hangs low enough to jump from a rock at the corner of the U-turn, where the path underneath narrows before going up hill.
Best solution: USE THE TANKS. All of them. Take one and park it up on the rock, get a second one, and steer it up the rock and close against the overhang without going back down hill. Hop back into the first tank and roll up onto the platform. Then go back down and get the third tank and take it to the platform like the first tank. Now stack those tanks and claim the skull.
Nah... sounds too simple. Get the tank! In fact, get both of them! Make one into a ramp for one tank to climb and the other to scale off of and get the skull!
You should try using a grav lift, Weapon storage container, and a brute chopper for that one hidden on that alcove over the rampway on route to the Cartographer. Its like a NASA experiment with all the fun and explosive damage when it fails!
ha, i did something like that to get that skull! If you ever get the time, go into forge and try to hit the top of the sandbox on Sandtrap. Here's a hint, plasma batteries, fusion cores, mines, grenades and a well placed sniper round while standing on top of it. (looks awesome in theater)
IWBYD was funny because of the dance involved. I think the hardest skull would be Cowbell, where you need grav-lifts and luck. For that one I relied on ingenuity and a NASA logic of using those covenant weapon crates in the room before it as a launch pad.