I always make heavy use of snipers. Deploy two snipers and take out all the enemy troops. Use lancers to take out all the machine guns, then use either the lancers or the Edelweiss to knock down all four ruin walls (don't destroy them). The Batymus will fire at the first one to clear a path, at which point you should climb up the ladders and maneuver lancers or scouts to destroy the radiators (scouts can drop grenades in for a lot of damage). Repair the Edelweiss with an engineer (taking cover behind it) whenever necessary. The engineer should also resupply lancers when necessary.

After the second radiator is destroyed, Selveria will arrive with a new force of troops. Move everyone (except one sniper; you can have the other sniper retreat) as fast as possible behind the most southern walls and hide them there. Move the sniper to the top of the hill (keeping cover) and take out all the troops but Selveria. When they're all dead, have him retreat.

As Selveria nears, block her advance by wedging the Edelweiss next to the Batymus and in front of the descending ramp. She can't damage the Edelweiss. As the Batymus moves, continue to block her path, but save as many command points as possible. Once it fires, sneak a scout or engineer out to destroy the final radiator (this should probably be the one on its starboard side). Then have your lancers move out from their hiding space and use as many turns as it takes to destroy it, resupplying with the engineer.

Obviously, save frequently over multiple saves while attempting this.

Mostly because the first time through I missed the first shot against the tank guns with my Lancer and assumed that this meant it was invulnerable. I was really pissed on the second try when I found that the Lancers could hit it.

It took me about 10 tries to figure out the sneak mission with Alicia and Welkin, saving would have been helpful there!