Level 28

Total Experience Required 109,938 [confirmed]

Level 100 progress: 1.61%

This is roughly:
Killing 367 people in Warzone, or...
Killing 374 people in Execution, or...
Killing 311 people in Wingman, or...
Scoring 78,528 points in Guardian, or...
Scoring 238,996 points in Annex, or...
Scoring 144,656 points in Submission.

Horde (TU4)

Completing 550 waves on Casual, or...
Completing 367 waves on Normal, or...
Completing 275 waves on Hardcore, or...
Completing 220 waves on Insane.

Or a combination of the above. Have fun!