Start "Arctic Recon" on regular difficulty as this is very hard to do under 3:30 on a harder difficulty.
Don't waste time picking up a weapon at the beginning as it isn't necessary.
You will want to have played this before to know where the beacons are placed.
Run to each beacon location, take cover and use the recon drone first to tag each objective, take out any enemies that are shooting you, don't waste time on killing everybody. Plant the beacon and move onto the next one.
Save your flash bangs until the run back to the chopper to blind everybody and just run past them.
Video below for help.