Kill enemy commander
Have an non-destructable bunker with a high ranking officer inside that needs to be taken out. Players would have to go in to the bunker on foot to take him out. No way should be given to take him out from the outside, neither air, neither snipers, neither any form of anything thats not inside the bunker. Maybe add a few external positions to specifically protect said bunkers, like smaller bunkers around it and so on. These additional extras around the main bunker however can be destroyed either by CAS, or snipers, etc… you get the idea.