Level 66

Total Experience Required 1,665,516

Level 100 progress: 24.4%

This is roughly the same as:

Killing 2,776 people in Warzone, or...
Killing 2,833 people in Execution, or...
Killing 2,353 people in Wingman, or...
Scoring 594,828 points in Guardian, or...
Scoring 1,810,344 points in Annex, or...
Scoring 1,095,735 points in Submission.

Horde (TU4)

Completing 4,164 waves on Casual, or...
Completing 2,776 waves on Normal, or...
Completing 2,082 waves on Hardcore, or...
Completing 1,666 waves on Insane.

Or a combination of the above. Have fun!