Take A Spin On The Scarab...:

On the mission 'Metropolis' just after you beat the wraiths and are about to go up to fight the scarab, grab the warthog that arrives soon after.

Take the warthog and park it just out side the first set of small stairs. Kill all of the marines in the area and then use the M68 hypersonic, 25mm shell firing gauss cannon on top of the warthog to clear out the crates just up from the stairs (and any other angry marines still left around). Now (with a bit of patience) guide the wart hog up the stairs. Keep going up until you are outside just in time to see the scarb incinerate a scorpion. Continue up and then after you hear Johnston scream "that thing is REALLY STARTING TO PISS ME OFF!" continue through the door, up more stairs and then off the nearest balcony into some poor bewildered elite patroling quietly aboard the scarab.

You may have to wait for the scarab to stop at before you make your stunning entrance.

I think they should encorporated this idea into their game... Its a very heroic way to board an enemy vessel in my opinion.

Enjoy!

P.s: You wont be able to get down to the cockpit with your vehicle. you can aline it though so that you have a clear shot with that scary sound splitting shell ejecting contraption on top of your warthog... the elites dont seem to like that to much.