Raiola will accept nothing but the best terms for his client and himself and Arsenal aren't exactly a club notorious for indulging. I think this will go on for a long time, only for both players to end up staying where they are. Unless Arsenal accept a straight cash offer, which seems unlikely as they seemingly only want to do business if they have a replacement.
Then again, I've been wrong constantly about this saga, so feck if I know anything. I'm just a random idiot.
He doesn't get the best terms for his client, he gets the best possible terms.
And the problem with the deal falling apart is that his client sits on the bench for the next 6 months losing value (especially if Sanchez comes for straight cash) while Raiola misses out on a fee. He's not an idiot, he's not gonna let this happen unless the deal offered by Arsenal is really bad for Mkhi in which case it wasn't going to happen anyway.
Like every good negotiator he will string this out and play hardball all the way up to the point where the other side is ready to walk away form the negotiations. Then he will strike a deal with the best possible terms.
Just have patience. It's best for all parties involved, including both players and their agents, to see this through. He will make it happen.