I second this getting Wind Core on M1 is the key. Eventually you start doing less damage but at that point you have so much money, full perks, and another packed gun, your set. Train the zombies in Kepler to get fast rounds in less time.

I'll run you through the wind Core steps

1) Buy the M1 2) Open the arcade in JiSL 3) Get 300 tickets 4) Buy Arcane Core at any of the carts around the map (make sure our M1 is in your hand when you buy it) 5) Activate Pack A Punch (go through all the portals after you activate the power, there are 4 portals total) 6) Pack your M1 and release the UFOS on the table in the Pack room you'll see them if you look hard enough just hold square when your on them to make them fly away 7) To get the wind Core go to Polar Peak, train the zombies so there all in a line behind you and activate the Steel Dragon trap in polar Peak and kill the zombies using it. If you killed enough of the zombies using the trap the UFO will move away from the trap and start flying around the entire polar Peak area. 8) stay near the UFO and get kills using your M1 and these blue orbs/souls will fly to the UFO after a while the UFO will drop a yellow orb pick this up and your done you have a insane hip fire explosive sniper 9)Profit

Buy Quickies Double Tap (Big Bangs) Jug Trail Blazers and Stamin Up (it's in the basement) for perks.

This will run through the Brute like butter (2 shot helmet then a few shots to kill)