Level 66

Total Experience Required 1,665,516 [confirmed]

Level 100 progress: 24.4%

This is roughly:
Killing 5,552 people in Warzone, or...
Killing 5,666 people in Execution, or...
Killing 4,705 people in Wingman, or...
Scoring 1,189,655 points in Guardian, or...
Scoring 3,620,687 points in Annex, or...
Scoring 2,191,469 points in Submission.

Horde (TU4)

Completing 8,328 waves on Casual, or...
Completing 5,552 waves on Normal, or...
Completing 4,164 waves on Hardcore, or...
Completing 3,332 waves on Insane.

Or a combination of the above. Have fun!