The Lighthouse really is the key to this battle. No matter which faction you play, your very first order of business should be to capture it.
As Imps - I spawn as a Heavy at the AT-AT and run to the Lighthouse. Along the way, I toss out one grenade toward the Fortress, which usually catches scum spawning from there. Once the Lighthouse is captured, I lay mines along the north stairs while blowing up their Speeders. Sometimes I switch to a regular Trooper or Sniper to protect the Caves entrance, though mines work wonders there too. Really, though, all you need to do is take and hold the Lighthouse. With only 2 CPs left, the Rebs die of attrition.
Speeder note: If you have a chance to take a Rebel Speeder, do it! Thrust directly toward the ice wall between the Citadel and the Fortress - you'll go partway up the wall, and can land on either the Citadel or Fortress and blow away thier turrets and fresh spawn. If you get grenaded too heavily, just drop down to the R5.
As Rebs - I spawn as a Vanguard at the Fortress, run to the Lighthouse (dropping mines along the way), stand next to the Gonk and fire missiles at the AT-AT. Once they lose the AT-AT, they lose reinforcements until it respawns. Again, easy win through attrition, though I can usually take the Ice Caves later in the game too.
As Seps - Stand there and twiddle your thumbs and let the Droidekas do the heavy lifting. Seriously, these things are a bit overpowered in this game. If you want to actually contribute, follow the Reb strategy - spawn as an Assault Droid, lay mines around the Lighthouse and rocket the crap out of the AT-TE. Your Droidekas will often take the Ice Caves for you while you're doing that.
As Republic - I have a horrible time dealing with Droidekas, even as a Jet, so I rarely play the Clowns. Same general idea though - hold the Lighthouse, defend the Ice Caves entrance, and let attrition work its magic.