Level 55

Total Experience Required 869,952 [confirmed]

Level 100 progress: 12.75%

This is roughly:
Killing 2,900 people in Warzone, or...
Killing 2,960 people in Execution, or...
Killing 2,458 people in Wingman, or...
Scoring 621,395 points in Guardian, or...
Scoring 1,891,200 points in Annex, or...
Scoring 1,144,674 points in Submission.

Horde (TU4)

Completing 4,350 waves on Casual, or...
Completing 2,900 waves on Normal, or...
Completing 2,175 waves on Hardcore, or...
Completing 1,740 waves on Insane.

Or a combination of the above. Have fun!