Level 51

Total Experience Required 669,305 [confirmed]

Level 100 progress: 9.81%

This is roughly:
Killing 2,232 people in Warzone, or...
Killing 2,277 people in Execution, or...
Killing 1,891 people in Wingman, or...
Scoring 478,076 points in Guardian, or...
Scoring 1,455,011 points in Annex, or...
Scoring 880,665 points in Submission.

Horde (TU4)

Completing 3,347 waves on Casual, or...
Completing 2,232 waves on Normal, or...
Completing 1,674 waves on Hardcore, or...
Completing 1,339 waves on Insane.

Or a combination of the above. Have fun!