:: This can be done on ANY difficulty :: The scarab gun is in the Metropolis level and is the most powerful weapon in the game. First kill all of your marines on the bridge- if you see ANY marines, kill them. Get in the tank and blow up any ghosts or wraiths you see.
When you get to the banshees, kill all of them but one.(this is VERY important!) Kill the rest of the covenant and lure the banshe to follow you. Blow up the warthog full of marines and continue on with the banshee still following you. Kill all of the aliens in the next curve.
The banshee might be a little mental and get stuck in a corner, but just stand behind it untill it re-targets you. Have it follow you in to the small tunnel.(you might have to blow its wings off with a plasma rifle.) EXACTLY when it says "Loading... Done" you have to hijack it, and if you wait too long, the banshee will dissappear and you will be sad.
If you do this correctly, you should be in the banshee. Go out into the next outside area and you will see two gigantic bridges. Fly on top of one of them and you should see what looks like a plasma rifle floating on top of a cone. Grab it and when you shoot it should be a big plasma-like explosion.