This mission is in a northern California logging area, and is basically a giant loop.
Starting near the gas n' grill, head into the park and around the burning truck. You'll see some Sectoids and a Shield Commander. As you fight them, more Outsiders and a Muton will emerge from the spire's base.
Enter the spire, and like Carter says: top off your ammo. If you got an Engineer, put some mines along the far energy door. Poison the spire via the console, and be prepared to fight of an Elite Muton and several Elite Outsiders. There's a supply drop soon, so go ahead and pepper the Muton with Sticky Grenades (he'll land right on the center of the energy doorway, so aim to tag him as soon as the door drops. With him out of the way, you can now deal with the Elites now hounding you. The area is wide, so you're almost certain to get flanked; act fast.
When done, go out the back of the spire, and hit up the supply drop. Past the burning boat is a camp site and Outsider fortifications on the right. There is an Elite Sniper, Elite Phantoms, and Elite Outsiders. More Elites from a second Drop Pod-- a Recon with bombardment and the Range pack can take it no problem, and then a third, which will also spawn a Tech Commander, who will generate a Rocket Turret if you don't kill him fast enough. The Rotary Plasma Cannon is available to pick up in the Outsider structure. Through the door is a large energy Pack for your shiny new gun, and the Anti-Personnel schematics.
Head through, and you'll find yourself back at the gas station. If you still got that Engineer, now's a good time to lay down some more mines. Expect a horde of various Outsiders, three drop-pods, and a Gunship. Staying on the roof keeps you out of melee range and decent cover from those Elites, but exposes you to the Gunship. Going in provides cover from the ship, but is very porous defense from the aliens on the ground.