Level 90

Total Experience Required 4,923,103 [confirmed]

Level 100 progress: 72.12%

This is roughly:
Killing 16,411 people in Warzone, or...
Killing 16,746 people in Execution, or...
Killing 13,908 people in Wingman, or...
Scoring 3,516,503 points in Guardian, or...
Scoring 10,702,398 points in Annex, or...
Scoring 6,477,768 points in Submission.

Horde (TU4)

Completing 24,616 waves on Casual, or...
Completing 16,411 waves on Normal, or...
Completing 12,308 waves on Hardcore, or...
Completing 9,847 waves on Insane.

Or a combination of the above. Have fun!