Level 18

Total Experience Required 39,764 [confirmed]

Level 100 progress: 0.58%

This is roughly:
Killing 133 people in Warzone, or...
Killing 136 people in Execution, or...
Killing 113 people in Wingman, or...
Scoring 28,403 points in Guardian, or...
Scoring 86,444 points in Annex, or...
Scoring 52,322 points in Submission.

Horde (TU4)

Completing 199 waves on Casual, or...
Completing 133 waves on Normal, or...
Completing 100 waves on Hardcore, or...
Completing 80 waves on Insane.

Or a combination of the above. Have fun!