Hunters on Reach. Especially a close quarters duel, without any CQC weaponry, where you're forced to constantly close the distance with the hunter (in order to avoid the minimum range of their fuel rod gun), evade it's attacks, pummel it in the backside, and evade the retaliatory strike, all the while dodging supporting fire from the Hunter's allies, or perhaps fire from the hunter if you're not careful. I mean, it's where a single misstep lands you in either the hunter's stroke, or past the minimal range of the hunter's arm mounted cannon, both of which provide you with a pretty large chance of being screwed up the pie-hole. IMO, taking on a single hunter requires far more skill than taking on any other A.I single opponent.