Advance to the Core Ship
Locate Passage To Next Canyon
Now that you've killed half of the droid army almost single-handedly, your group's task is to infiltrate a disabled Separatist core ship. In order to reach it, though, you'll have to maneuver through a series of canyons on Geonosis' surface; there's a large anti-air turret that prevents your dropships from getting close to the ship.
Soon after you start your journey, you'll come to a sheer brick wall; assign a squaddie to blow it while the other two provide covering fire against the Geonosian attack. When that's done, you'll be able to make your way forward to a crashed dropship. You'll want to rush to this position as quickly as possible, since the laser turret on the ship will provide you with a heck of a lot of firepower when placed in the hands of one of your squadmates. (You can also use it yourself, if you wish, but you'll find that your squaddies are much better at using it than you are.) There are also two sniping positions near the ship, so try to make sure that all of your squaddies are positioned behind cover.
When they're all in place, all you really have to do is wait; eventually the influx of Geos will cease a bit. When you notice that no more bugs are appearing, heal up your soldier, but leave one of them in the turret and another in the sniping position near the ship before moving on. Your passage down the canyon will trigger another influx of bugs, which you'll be much more able to deal with if you have the sniper and turret on your side; just run up a bit, then retreat back to the dropship. There's going to be one Elite near the rock arch, so grab him, then kill the rest of the bugs before moving on to find another Elite. Again, lure this one back to the dropship for an easier kill before finally cancelling your squad's orders and forming up.
When you reach the next bacta dispenser, get one of your squaddies into the sniping spot, then order anyone who's wounded to heal themselves before walking up; doing so will trigger yet more Geonosians to appear, so keep killing them until the influx ceases. When this occurs, heal up again, then leave your sniper in position while the rest of the squad proceed to the end of the canyon and destroys the rock wall blocking the exit from the level. After it's been blown, you'll be able to access the mission end, so ignore any remaining enemies and run to it. Before you head down, though, make sure all of your squadmates are at full health, and that you have a full reservoir of ammo at your disposal.
Canyons of Death
Advance to Coreship
The opening to this level is going to be the scene of a pretty fierce firefight; all you have to your advantage is a single sniping spot, while the Geonosians have numbers and the ability to fly on their side. All you can really do is position one of your guys at the sniping spot, place the other two near him, then try and outlast the enemies as they come in. The real trouble here is the appearance of an Elite about halfway through the firefight. If possible, try to move a bit forward down the canyon before it appears, to get him to target you, then use your own Elite laser on him (if you still have one). It can be difficult to train your laser on him while attempting to avoid his, but this is really your best shot at dispatching him without letting the rest of your squad get killed by the other Geonosians in the area.
When the first little melee is over, start commanding your squaddies to run down to the bacta dispenser near the bend in the canyon. As they're healing, command any healthy teammates to man the two sniping positions nearby, as there'll be droids and bugs incoming shortly. Wait in your current positions until the flood stops, then re-heal everyone before you move on.
Keep in mind that only you need to reach the entrance to the AA bunker; you can leave your teammates behind, and they'll join you at the start of the next level.
Before you can make it to the Coreship, you'll have to eliminate the anti-aircraft bunker that's destroying your ability to get resupplied. Unfortunately, it is heavily guarded by multiple SBD's, so be cautious when rounding the corner that leads to it. It's probably best to leave your squadmates back a bit while you approach it, just so you can trigger its defenses and know what the heck is going to happen when you send your squadmates around. Actually, approaching it will also activate a pair of anti-personnel turrets above the entrance to the bunker, which is good, because you'll then be able to pick them out from the corner and snipe them out before you lead your squad through. You should attempt to snipe away any of the SBDs in the area, as well, before you move the rest of your squad up and into the grenadier and sniper spots along the walled corridor leading to the bunker.
When you have two of your squaddies in position there, you'll be able to take the third and round the bend leading to the bunker. Unfortunately, two more SBD's stand in your way, so you'll need to have either Thermal or EC grenades to dispatch them. When they're dead, though, the enemies will cease coming out of the bunker, allowing you to form up and prepare to head inside.