Level 99

Total Experience Required 6,626,736

Level 100 progress: 97.08%

This is roughly the same as:

Killing 11,045 people in Warzone, or...
Killing 11,270 people in Execution, or...
Killing 9,360 people in Wingman, or...
Scoring 2,366,692 points in Guardian, or...
Scoring 7,202,974 points in Annex, or...
Scoring 4,359,695 points in Submission.

Horde (TU4)

Completing 16,567 waves on Casual, or...
Completing 11,045 waves on Normal, or...
Completing 8,284 waves on Hardcore, or...
Completing 6,627 waves on Insane.

Or a combination of the above. Have fun!