I saw this answer. It makes me curious that why Dumbledore is afraid of Greyback, considering that Dumbledore is more powerful than Greyback.
It really isn't that surprising - Dumbledore is no longer (or not for much longer) more powerful than Greyback. He has been cursed, and the curse is slowly killing him. It is also weakening him significantly, as his health deteriorates.
Even if that weren't the case, Dumbledore fears being at the 'mercy' of the Death Eaters. He specifically names Greyback and Bellatrix because they are known for favoring slow, painful deaths for their victims.
Dumbledore knows he is not long for life, and prefers that his life end at Snape's want instead of (after hours or days of torture) Greyback's teeth/claws/wand or Bellatrix's wand.
The killing curse is a quick death, and Dumbledore knows he won't suffer. He also believes that having Snape kill him will help preserve the life of Draco.
In short, he knows that he will die, he simply chooses the way that will cause him the least amount of suffering while accomplishing the greatest good (in this case, solidifying Snape's reputation as a vetted Death Eater and saving Draco's life).
Because Dumbledore knows Greyback will kill him painfully.
Dumbledore tells Snape that it's better that Snape kill him since he knows Snape won't make him suffer, unlike some of the others who could kill him instead of Draco. He's not scared of dying, and in fact he fully intends to die, but he'd rather his death be a quick one than a slow and painful death at the hands of someone less merciful.
“You alone know whether it will harm your soul to help an old man avoid pain and humiliation,’ said Dumbledore. ‘I ask this one, great favour of you, Severus, because death is coming for me as surely as the Chudley Cannons will finish bottom of this year’s league. I confess I should prefer a quick, painless exit to the protracted and messy affair it will be if, for instance, Greyback is involved – I hear Voldemort has recruited him? Or dear Bellatrix, who likes to play with her food before she eats it.”
- Harry Potter and the Deathly Hallows, Chapter 33 (The Prince's Tale<)
He explains this to Snape when trying to remind him why he should be the one to kill him, so he'd understand that it's indeed an act of mercy since Dumbledore's death is inevitable, but it being painful can be avoided. For that, it doesn't matter that he's more powerful than Greyback - he knows he's going to die, but doesn't want it to be at the hands of a Death Eater who would torture him, or at the curse which will kill him slowly. In addition, he wants to remind Snape of all this so Snape would feel more at ease with having to do it,
The important point is that Dumbledore needs Snape to be the one to take his life, for the good of The Plan. It will cement Snape's position in Voldemort's organization, winning him (hopefully) a level of trust he might not be able to achieve otherwise. It's also possible he was planning for Snape to become the master of the Elder Wand by doing so.
Remember, at this point, Snape absolutely did not want to carry out this part of The Plan...what Dumbledore is doing here is to try and cajole Snape into following through by rationalizing it, framing it as a favor he would be doing for Dumbledore.
Dumbledore isn't necessarily doing this avoid Greyback or Bellatrix (though that is admittedly a strong bonus), but rather to overcome Snape's resistance to doing what needs to be done for The Plan to work.