Park a buggy in an unsafe location (such as a flight of stairs in the Fort level). Eventually a pig will get in and drive the buggy and park it back on the road. Note: The pig may run you over if you are in the way.
When facing the last group of mercs protecting Doyle at the top of the stabilizer during the final level, slowly approach the ramp until you hear the radio chatter confirming they have spawned in above you. After that, back off and look up for the large fuel tanks. You cannot see them, but the mercs are gathered around and behind these tanks. Scatter throw all of your grenades at the tanks to kill or wound almost all of the mercs, leaving just one or two to deal with upstairs.