Just back peddle and shoot them.
If they get to the point where they have to go down a hallway to eliminate you, make sure you have your shotgun/fusion/sidearm ready.
Remember, they chose this, not you.
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Added notes: Most players who use 750s are either too arrogant in 750’s or believe they can’t be stopped with one. Once you start to back up and shoot them at a reasonable distance, players will continue to push you because 750’s require the user to push for full usage. Most modes require kills. Just don’t get picked and you’ll be fine.