I will have to say By my hand alone and Daredevil, head hunter is just a matter of a steady hand.
By my hand alone
I spend 3 days on it. 5 hours the first night, 6 the next one and finally 4 the following. i think i practice enough to know how to fight each enemy i master killing every one from rounds 1 to 8. managed to get untouched from rounds 1 through about 5 or 7. I will just hide and grenade the crap out of round 8. i used bend time from rounds 9 on.
Head hunter is a matter of practice as long as you can get the one guy in the spiral stairs i will have to say i got lucky.
Finally Daredevil...Jeeeez that one is the tough one but it just takes a lot of practice.
It just got to the point that you play it enough that you just are naturally good at it. once i hit the 30k marker i just quit. did not even try to go further.
Just Keep at it...Respect for those that can be up there on the top of the leaderboard and those that can 100% the trials without failure. pretty sure if i play enough i can be good too.