Fight to the end, and give it your all.
First, I would have one Falcon and Warthog, filled with Marines, distract the Scarab and provide support for those boarding the Scarab. Then I would have the other Falcon, full of ODST'S board the Scarab equipped with Rocket's and standard gear. Meanwhile, I would have the sniper in a location where he could provide support for the ODST'S boarding the Scarab. As for the remaining Marines and ODST'S I would attempt to have a Falcon continually drop them off on the Scarab, or let them be a distraction. Whichever one that's more efficient.