Level 52

Total Experience Required 715,858 [confirmed]

Level 100 progress: 10.49%

This is roughly:
Killing 2,387 people in Warzone, or...
Killing 2,435 people in Execution, or...
Killing 2,023 people in Wingman, or...
Scoring 511,328 points in Guardian, or...
Scoring 1,556,214 points in Annex, or...
Scoring 941,919 points in Submission.

Horde (TU4)

Completing 3,580 waves on Casual, or...
Completing 2,387 waves on Normal, or...
Completing 1,790 waves on Hardcore, or...
Completing 1,432 waves on Insane.

Or a combination of the above. Have fun!