Video Walk-through
If you didn't happen to have a Wookiee Rocket Launcher for this spider droid, it's still killable - it'll just be really tough.
More SBD's and droidekas await you beyond Fixer's turret. You'll get a resupply of AA and EC ammo, though, of course, so you should be able to make it down the ramp without a problem. After the curve, in fact, you can start shooting the explosive barrels off the walls to start a little chain reaction that'll kill off most of the droids without requiring you to fire at them.
Beyond the next door is...another spider droid??? Wow, with only one partner along to help you, you can expect to face a major challenge in taking this guy out. To make matters worse, you're going to have Geonosian soldiers along for the ride; they'll spawn in in pairs and harass you and your partner. You're going to have to use every trick in the book to survive this one.
To begin the fight, open the room, shoot the fuel tanks to the left, and set your soldier up in the anti-armor position there while you take out the droid's scanning laser to prevent it from firing missiles. At this point, a long game of cat and mouse will start to play out. In the arena here, you'll have refills of thermal dets, sonic grenades, anti-armor rounds, sniper ammo, and blaster rifle ammo. Before you start picking it up, though, you'll want to start rotating around the spider droid and unloading whatever ammo you brought into the fight on it, so that you don't waste any when you grab the ammo refills. Your goal is to distract the droid and hopefully cause it to indefinitely rotate around as it tracks you; this will let you keep chucking grenades and rounds at it while letting your teammate do the same from his position. It's important to keep your teammate alive, though, since you won't have a chance to resurrect him if he dies, so periodically task him to the bacta dispensers in the corner. By the same token, if he's alright, but the tank is focused on him, feel free to make a pit stop yourself, but be careful of the Geonosians if you attempt to do so.
When you're out of ammo, start grabbing the boxes around the arena and repeat the process again. We're not saying this is an easy fight--on hard mode, it'll be among the most difficult in the game. If you happen to have kept the Wookiee Rocket Launcher from the first part of this level, then by all means, use it now and put this thing out of its misery. Otherwise, you'll just have to wear it down and hope it doesn't kill you.
Moving On
After breaching the next door, pause for a second for the explosives on the other side to go off before entering with your nightvision goggles and slaughtering the Geonosians inside. Be sure to pick up the Wookiee Rocket Launcher near the far door before moving out, as yet another spider droid awaits you. Luckily, the WRL you just got will take off the bulk of its health, allowing you to place your squaddie in the anti-armor position near the door and whittle off what remains with AA rounds and grenades.
The Final Charge
After you get through the next door and get Scorch into his turret, it'll be time for you to find yours and destroy the Separatist ship once and for all. Heal up and grab the sonic detonators before breaching the door near Scorch's turret.
The room here has been taken over by the Geonosians and has apparently been used as a breeding ground; there are multiple egg chutes suspended from the ceiling, along with a few Geonosian soldiers. You shouldn't have to worry about them too much, though; if you just keep moving, you'll be able to avoid the fire from the spawn and get to the far door without too many problems. When you do so, turn around and start chucking sonic grenades behind you; this will disintegrate most of your foes, save for those standing around near your turret.
When everyone's dead, grab the turret and start firing away. There are three separate spots on the Separatist ship that you'll need to hit in order to destroy it, so use the targeting cursors and go to town! You have unlimited ammo, so just keep firing until the cruiser is all blown away.
When that's done, well, so is the game. An ambivalent ending leaves a few questions unanswered: What was Grievous doing on Kashyyyk? What fate awaits the Wookiees? Guess we'll just have to wait until Episode III to find out.