Level 99

Total Experience Required 6,626,736

Level 100 progress: 97.08%

This is roughly the same as:

Killing 22,090 people in Warzone, or...
Killing 22,540 people in Execution, or...
Killing 18,720 people in Wingman, or...
Scoring 4,733,383 points in Guardian, or...
Scoring 14,405,948 points in Annex, or...
Scoring 8,719,390 points in Submission.

Horde (TU4)

Completing 33,134 waves on Casual, or...
Completing 22,090 waves on Normal, or...
Completing 16,567 waves on Hardcore, or...
Completing 13,254 waves on Insane.

Or a combination of the above. Have fun!