You'll most likely earn this before finishing the 30 levels. Kills are accumulative
Achievement Walkthrough:
*Timestamps*
Level 1 - :20
Level 2 - :40
Level 3 - 1:10
Level 4 - 1:42
Level 5 - 2:00
Level 6 - 2:17
Level 7 - 2:33
Level 8 - 2:53
Level 9 - 3:10
Level 10 - 3:45
Level 11 - 4:15
Level 12 - 4:38
Level 13 - 4:52
Level 14 - 5:05
Level 15 - 5:23
Level 16 - 6:19
Level 17 - 6:40
Level 18 - 6:52
Level 19 - 7:20
Level 20 - 7:49
Level 21 - 8:10
Level 22 - 8:41
Level 23 - 9:02
Level 24 - 9:18
Level 25 - 9:37
Level 26 - 10:01
Level 27 - 10:18
Level 28 - 10:42
Level 29 - 11:14
Level 30 - 11:38