This can be an annoying achievement, especially due to the randomness of the AI spawning monsters... But here is a helpful hint - The achievement can be popped by not only necessarily killing 10 monsters in a row by shooting them, but just successfully landing 10 shots in a row.
Make sure in the options the auto aim thingy is ON, and there are a few monsters that require more than one shot to kill, and will all add up to the achievement, making it a little easier.
For example, I got this by shooting one monster with a single shot, then by killing the insect looking monster which flies upwards and zigzags which takes 5 shots to kill. This put me on 6 shots in a row. I then ran into another one of these insect monsters and killed it with another 5 shots, unlocking the 10 in a row achievement.
Hope this helps - Also applies for the 5 shots in a row achievement.