Level 26

Total Experience Required 91,218 [confirmed]

Level 100 progress: 1.34%

This is roughly:
Killing 305 people in Warzone, or...
Killing 311 people in Execution, or...
Killing 258 people in Wingman, or...
Scoring 65,156 points in Guardian, or...
Scoring 198,300 points in Annex, or...
Scoring 120,024 points in Submission.

Horde (TU4)

Completing 457 waves on Casual, or...
Completing 305 waves on Normal, or...
Completing 229 waves on Hardcore, or...
Completing 183 waves on Insane.

Or a combination of the above. Have fun!