Level 40

Total Experience Required 302,351 [confirmed]

Level 100 progress: 4.43%

This is roughly:
Killing 1,008 people in Warzone, or...
Killing 1,029 people in Execution, or...
Killing 855 people in Wingman, or...
Scoring 215,965 points in Guardian, or...
Scoring 657,285 points in Annex, or...
Scoring 397,831 points in Submission.

Horde (TU4)

Completing 1,512 waves on Casual, or...
Completing 1,008 waves on Normal, or...
Completing 756 waves on Hardcore, or...
Completing 605 waves on Insane.

Or a combination of the above. Have fun!