To gain this achievement you need to earn 200,000 kills with one weapon using manual touch targeting. The weapon levels are displayed below.
Level 1: Given From Start
Level 2: 50 kills
Level 3: 250 kills
Level 4: 2,500 kills
Level 5: 50,000 kills
Level 6: 200,000 kills
The number of kills is ONLY from touch targeting, in other words, manual targeting. This is when you place the cross hairs either on a mission vehicle or some specific place on the map or when you manually aim/shoot a gun by touching it.
One easy way is finding a long stretch of road and setting up lasers. I manually set the targets so they would shoot down the road and destroy many freaks in one shot.
Their are plenty of long roads where you can set up the specific weapon that you using and set targets along the roads for them to shoot. Kill hordes of enemies as they appear - there are plenty of roads like this in Australia.