In rampage four you have to kill 35 military personnel within 3 minutes using grenades, an assault rifle and a grenade launcher.
Use the grenade launcher to blow up the vehicles as they come towards you which will kill the troops inside.
To destroy the tanks climb the hill opposite the bridge and use the hill as cover so the tanks can’t hit you. You could also use the debris as cover or strafe to avoid the tank shells. Each tank takes about 5 shots of the grenade launcher to blow up.