Level 25

Total Experience Required 82,884 [confirmed]

Level 100 progress: 1.21%

This is roughly:
Killing 277 people in Warzone, or...
Killing 282 people in Execution, or...
Killing 235 people in Wingman, or...
Scoring 59,203 points in Guardian, or...
Scoring 180,183 points in Annex, or...
Scoring 109,058 points in Submission.

Horde (TU4)

Completing 415 waves on Casual, or...
Completing 277 waves on Normal, or...
Completing 208 waves on Hardcore, or...
Completing 166 waves on Insane.

Or a combination of the above. Have fun!