Not much to add the other solutions, however, I found those not very clear. Here are some details you might find helpful to know.
Level: Act 1: training (casual)
Average duration: 2:30
Average kills: 81
Estimate length of time for getting achievement (from fresh start):
5 min = 162 kills, 60 min = 1944 kills
10.000 : 1944 = 5,14 hours = 5:08:00
Position you should take within the level:
Above the left side of the Big Ben (the clocktower) always spawn enemies. Take your position AT the upper spawn, and just shoot down the whole match. Don't bother taking any control points.