Provide covering fire, provide assistance when a teammate is pinned, and find the right strategy for the urban combat you'll face. 
Call in the Marine Air Ground Task Force (MAGTF) -- get air support for Cobra helicopters and ground support for M1 Abrams tanks. 
Make sure you follow all rules of engagement -- the biggest challenge any Marine can face. 
For added challenge, you'll have to manage the psychology of your teammates - make sure confidence stays high and avoid giving orders that are questionable. 
Two very different modes of play - More casual, traditional shooter action in Normal Mode or try Simulation Mode - where enemies are more precise, and teammates will take on more damage.