Well, in Geonosis, the only hard part that stands out in my mind is the beginning of one level where you're in the canyons. You just walk forward a bit and GEONOSIANS. THOUSANDS OF THEM. start coming out. Took me forever to kill them all, and usually the whole squad was out by then.
The RAS hard parts were the level Dues Ex Machina and Live Ordnance when you clear out the hangars, there were just all those droids. In Live Ordnance I just ran to the back and set Sev and Fixer on sniping positions and hoped Scorch stayed alive till the AT-TE came up. Then I held the droids back with EC grenades and anti armor.
On Kashyyyk, the hardest part that I remember was the Kachiro bridge that had those two SBD dispensers, the hordes of battle droids, the turret...and after you clear it all out there's a Spider Droid. For the first part I would clear out the droids on the bridge, set someone at a sniping point, someone on a turret, then cover someone else as they destroy a SBD dispenser.
My sig is stupid.