Level 88

Total Experience Required 4,573,128

Level 100 progress: 67%

This is roughly the same as:

Killing 15,244 people in Warzone, or...
Killing 15,555 people in Execution, or...
Killing 12,919 people in Wingman, or...
Scoring 3,266,520 points in Guardian, or...
Scoring 9,941,583 points in Annex, or...
Scoring 6,017,274 points in Submission.

Horde (TU4)

Completing 22,866 waves on Casual, or...
Completing 15,244 waves on Normal, or...
Completing 11,433 waves on Hardcore, or...
Completing 9,147 waves on Insane.

Or a combination of the above. Have fun!