Have at least one engineer with Welkin, and a sniper. The rest can be whatever I chose a lancer. The first part will mostly be Welkin doing the work.
On the South team, have Alicia, Rosie, Largo, another shock trooper or two, and another Lancer. make sure you have at least two lancers in total.
Turn One:
Move Welkin to intercept the tank. Take out the front turret, and the east turret (if possible). Go back into original position
Take Alicia, and grenade the wall just north of the second base. Alternatively, use a shocktrooper to do that as he's moving in to defend the second base.
Turn Two:
The tank should start moving, and should end up directly in front of Welkin. Take out the two north facing cannons. Use your sniper to take out the Lancer up on the cliff edge if he's already targeted Welkin. Use your engineer to heal up Welkin.
Move your scout and shock trooper into the second base.
If you can spare it, use your lancers to take out the south turret (the one in the front, if Welkin hasn't already done so).
Turn 3:
The tank will take out the first base at this point. Have Welkin take out the rear turret, and the last south facing turret (rear side). Start moving your Lancer southward.
Your scout and shocktrooper should be taking care of the on rushing troops without you needing to interfere.
Turn 4:
The tank will use his energy cannon to take out the first wall. Move your Engineer to left ladder on the tank and drop a grenade in. At this point, most of the enemy troops will have all been defeated too. Move Alicia to grenade the wall east of the 2nd base. Move your shocktrooper back to the trenches.
Turn 5:
Bombing the second radiator should be the last thing you do in this turn.
Move EVERYBODY except the sniper and your north lancer to the north side of the wall so that the hill and trenches are clear. Move Welkin to the small wall on the east and have him back up to it so the radiator is completely shielded by the standing wall.
Use a high movement character (I chose Engineer) to bomb the rear or right side radiator. After cutscene, move your engineer back into safety and huddle with the rest. make sure Alicia has enough room to move out.
Turn 6:
Brace yourself, here come the troops! By this time, Selveria should be hovering around on the south side of the wall, and a lancer should have revealed himself. Your north Lancer may have taken some decoy damage from Selveria too.
Snipe off the lancer if possible. If not, that's okay. Move Welkin to the west side of the wall everyone is cowering behind. Have him face south. He'll be blocking Selveria's progress this way, and will be your turret for the gambit. Hold position. You can use Rosie to peek outside the wall to take out oncoming troops, or use Welkin's Mortar to kill anything oncoming. It should be safe at this point to move your sniper to the remains of the 1st base, and snipe out any troops that have passed Welkin.
Turn 7:
The tank cannon should have fired at this time. Clear out any remaining troops, and then send in Alicia to board the tank and take out the last radiator.
At this point, focus your two lancers and Welkin to take that sucker out! When you run out of ammo, use your engineer to restock them up. You should be able to finish it off in this turn.
