Level 20

Total Experience Required 49,655 [confirmed]

Level 100 progress: 0.73%

This is roughly:
Killing 166 people in Warzone, or...
Killing 169 people in Execution, or...
Killing 141 people in Wingman, or...
Scoring 35,468 points in Guardian, or...
Scoring 107,946 points in Annex, or...
Scoring 65,336 points in Submission.

Horde (TU4)

Completing 249 waves on Casual, or...
Completing 166 waves on Normal, or...
Completing 125 waves on Hardcore, or...
Completing 100 waves on Insane.

Or a combination of the above. Have fun!