It's a piece of piss. You just need to knock over the wall ruins (tank or lancer) to force the batons to use it's main gun and then expose the cooling radiators for a scout to lob a grenade into. But you do need to take out the machine gun turrets or your scouts will get shot to pieces when they try to climb the tank.
Once you know how it is easy to get an A mission rating. The important thing is to keep people away from selvaria once she shows up, she hurts and is indestructable so avoidance is your only option.
Edit sorry didn't read completely. If the main gun cooling radiators are not all being wiped out with a single grenade hit to each one then there is something glitchy about the game I think. Final radiator for the tank engine to disable the Batomys needs to be taken out by lancers and your tank.
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