If it's the one I think it is (where the rebs and wookies have to capture the thing from the sith temple) you can't build anything, you have to use tactics instead of brute force, as DMUK said. It's tricky and took me several goes to get it right but it can be done.
Most important thing is to follow the advice it gives you and split your forces in 2 and attack from 2 different directions. I sent all the Wookie artillery along with about half the MDs and some troopers to make the diversionary attack on the gate at the right side of the imperial base. Everyone else went round the left of the map and attacked from the southwest. You need to time the attacks so they hit at about the same time, or maybe the diversion slightly sooner. Put the pummels on the command centre to get it out of the way and get Echu in a MD and take him to the compound with the bomb droids. Unload him and get him to convert them straight away then use them to knock out the turrets. Once you've crushed all resistance in the outer part of the base, send the pummels and anything else you've got to knock down the temple. Just ignore the fortress as you haven't got enough stuff to take it out. Once the temple is destroyed, the holocron thing appears and Echu can pick it up. If all goes well, the diversionary attack should have destroyed the gate and the turrets on the other side of the base, so Echu can escape that way and go straight back to the ships.
If that wasn't the one you meant, I've just wasted my time.