Properly someones said this before, its simple affective can work at heroic if your lucky, scroll down to skip 3 minutes of wasting your life.
if its been done, is in wrong forum, just plain sucks, lock/Remove/Delete/Move as you please.
'Scuse the typos aswell am tired x_x
Just a small guide not saying it will help ALOT but its affective. First of all you need is a level 3 tech level and the rest supply pads (pereferebly a barracks or vehicle depot to help ease base attack pain, but just a single one) set the rally point behind your base or lock your base, know upgrade the cannon on the 'hog to the gauss.
Keep all your default units, and start building warthogs, 20+ to start, go round and destroy ALL the nodes so the scarabs head turns extremley slowly, then rebuild up to as many warthogs as possible, the default scorpion, forge, and the other 2 rhinos all stay at base.
For the assult make your way to the head while its on the opposite side to witch your attacking, around 20+ warthogs attack the scarab, as the sight on the scarab gets halfway (so its straight) run all your hogs through the beam ASAP, you should only kill 1 -2 hogs if your unlucky,repeat this using the mac to wear it away slightly ( Not if your attempting it on legendary it dosent affect it at all) within 5 minutes it should die.
To get the node things, Just run in with 5 - 15 hogs and perhaps a mac, if its heavily defended or on legendary drop a mac in there then run away, only to return and fail. use the macs wisely, itll take a while to recharge, use this time to make hogs.
Laymans terms somehow: Make 20+ hogs with gauss cannons, destroy all the nodes which make the head turn quickly, make more warthogs, go in put all units at one side, when the beam starts coming to you drive as fast as possible through the light to the other side shoot, rinse, repeat. <- JUST READ THIS PART.
If you need help doing anything on campain, tell me we can do coop and ill show you it.
For the skull: Camp at your base and attack locust as they come in, or just patrol around while getting the generator/node things and itll apear North-west of the map, Kuh-Ching.
BLLLLLLAAAAAARGHHHH :D