Necromunda: Hired Gun

You can easily grind it either on the arena in Maryr's End (by the way, you can grind the achievement for shooting enemies while wallruning there, with infinite spawn of basic level enemies there. When you run out of bullets just exit and go up the monitor just outside the arena, it'll replenish it) or in mission. For me, it was at the beginning part of Koloss 44 - lots of space to wallrun and no enemies to interrupt.