Mechanically speaking, if you have the ability to deliver a 3 count deathstrike, you have the ability to deliver the Inflict Annihilation. It will only stick to Xenos as it is a Bane effect.
If you're "doing it right," this should only happen after a 1 on 1 Challenge has occurred, or after the grand melee that is possible but discouraged. That said, nothing is stopping you from finding your buddy on the field and eating his brain when nobody's is watching.
Well. Nothing you know about, anyway...