One of the hardest achievements on Halo that I ever accomplished was the ‚They came to hear me beg‘ achievement on Halo: Reach. The description of the achievement reads as followed:
Performed an Assassination against an Elite to survive a fall that would’ve been fatal.
So in order to complete this achievement you have to jump of the cliff at the start of the level Pillar of Autumn. Below the cliff are 3 Grunts and 1 Elite. You have to sprint to the edge of the cliff and then jump down trying to land on the Elite and perform an assassination.
It is tough. Oh damn is it tough. Well at least for me. I remember trying it for over an hour. Sprint, jump…fail. Haha and Emile in your head saying: Race you to her.
When I finally managed I was so happy! I jumped up and screamed. More of a roar if I am honest. It is such a satisfying feeling when you beat a tough achievement.