You can only use laser/plasma and minigun/cannon turrets on a planet. To take down POI turrets you really need direct fire weapons - preferably railguns so you need to use an SV or HV (HVs are significantly better in RE2 - take a look at XCal's light tank). Note also that you can take blast damage through shields in RE2. Flak cannons and Arty do a large area damage through shields whilst rail guns do a lot of point penetration through shields. Plasma does some damage through shields and medium damage to shields.