Time to take the jocks down a peg or two. Petey tells you you'll need the nerds' help and to acquire that, you'll need to talk to Earnest. But where is he? There's a couple nerds hanging around outside the library. Ask them nicely about Earnest's whereabouts.
Now ask not nicely. Just don't knock him out. Unconscious nerds can't spill the beans. Oh, he's at the observatory. Why didn't you say so?
Grab the ruler that's lying on the ground, hop the wall into the alley beside the library, and beat the gate code out of Thad. Go through the gate and hop the barricade. Take out the Firecracker chucking nerds from afar with your charged up Slingshot. Hop over the next barricade and repeat. Watch out for the nerd hiding behind the rock on the right. He might sneak up behind you. Keep your eye on the radar so you know where everyone is.
Mosey on into the next area and take out the snipers. Just keep hitting the Lock On Button to locate threats quickly. Soon you'll approach the main gate of the observatory.
Okay, Earnest has been watching too much American Gladiators. Work your way up to the gate using the columns as cover from the Spud Cannon. Move when Earnest stops to reload. Make sure you take out the two nerds by the gate. Once you're directly under the Spud Cannon, Earnest can't hit you. Look to the left of the Cannon and you'll see a transformer with a glowing red light. Hit that with your Slingshot.
The observatory gates are now open. Go inside, run up the stairs to your right and man the Spud Cannon. Time to give the nerds a taste of their own medicine. Concentrate your fire on the observatory's front doors. Nerds will take pot shots at you from windows and the balcony but they don't do a lot of damage. If they're bugging you or your health is low, turn the cannon on them. When the doors buckle under the pressure of potato projectiles, abandon the Cannon and go on in.
Amazingly, Earnest still doesn't want to be your friend. Force him to see the light. Hide behind a column while he shoots his spare Spud Cannon at you. He reloads after four shots. When he does, pop out and shoot the generators behind him. They'll take about three shots each.
When the generators are destroyed the catwalk collapses and Earnest moves to another area. He's lost his Spud Cannon but he has double fistfuls of Firecrackers to hurl at you. Dodge the explosives as best you can and shoot out the two new generators quickly. Get Soda out of the crates lying around if you need it.
Two more generators down and one more catwalk destroyed. Earnest is on his last catwalk but he's found his Spud Cannon again. Hide behind the columns and pop out to shoot the generators when he reloads after every three shots. Destroy the final two generators and Earnest will give up the fight and give you his Spud Cannon.