Level 21

Total Experience Required 55,277 [confirmed]

Level 100 progress: 0.81%

This is roughly:
Killing 185 people in Warzone, or...
Killing 189 people in Execution, or...
Killing 157 people in Wingman, or...
Scoring 39,484 points in Guardian, or...
Scoring 120,168 points in Annex, or...
Scoring 72,733 points in Submission.

Horde (TU4)

Completing 277 waves on Casual, or...
Completing 185 waves on Normal, or...
Completing 139 waves on Hardcore, or...
Completing 111 waves on Insane.

Or a combination of the above. Have fun!