First you need a sniper.
Step 1: Get in Your Banjee
Walk over to the banjee of either base and get in.
Step 2: Head Towards the Highest Cliff
Go towards it and you will hit a wall (invisable of course)
Step 3: Right Next to the Gap
For this step, you will need to ram into the wall beside the giant gap, (please note: this will only work on this side, if you try it on the opposite side, you will fail.)
Step 4: Backing Up, Backing Up, Backing Up...
Now, this step may be rather difficult, and require practice, from the wall you get the back of your banjee to get in the gap, and as close to the wall as possible. Continue backing up, and DO NOT LET GO AT THIS POINT.
Step 5: On the Sniping Clif
Now, if you're still backing up on the cliff, get out of the banjee. Note: if you let go of the backwards button even ONCE... you will fall, now, eject out of your banjee.
Step 6: Proceed Sniping
You are now on the sniping cliff. Note: Helpfull items- Get more ammo for you're sniper. And a rocket. (for the other people that may try to get on the cliff, you can blast them to bits) or you can use a shot gun or plasma cannon. (your choice...personally i choose rocket)