How I did it:

1.Start the Gauntlet Challange
2.Skip to the 5th level "Unarmed Bridge Sprint", using the "theresnoplacelikeoz" cheat (It does NOT disable achievements)
3.Move to the right,bottom corner of the screen
4.Select your 3th weapon (Tactical shotgun)
5.Shoot all your shells at the turret north from where you stand
6.Suicide
7.Repeat steps 3-6

This works because turrets count as targets, and the ones in the Bridge Sprint are invulnerable to bullets/shells, so you're free to shoot at them untill you run out of ammo or the timer expires

Just put on some music or a podcast and grind it out

Have fun!