Mils are a lot less likely to "know" what is shooting at them if you are in a vehicle.

The best way to catch Mils that are already hunting you down off-guard is to throw a coring charge around a corner. Even better, in the case of the bunkers in the Access The Ruins level and Secret Rendezvous level, stick your head in, throw a coring charge, and get out. The results are amply satisfactory.

Always reload your weapon when you are done with a pack of Mils. Reloading in a firefight can be very costly.