Don't waste time on the Rhazin fusion until you have all the pieces. It could very well take you months, or even years if you're terribly unlucky, to get them, since it's completely random which rares you get.

About Rhazin... If i fuse him before the mission is up, will it count? Cuz you cant' fuse hm twice, so, he's not like the Relickeeper fusion is he?

Retroactive. 

And to OPs question: would stay focused on food and making more impactful 6*s, like Dreng. kram is correct in that it could be quite some time before you have the champs to finish off the fusion. 

Ive been 1 rare away from him for over a month easy , so no you should not focus on it until you have every single piece (and ideally during a CvC or champ chase)

Another option to consider is to not fuse Rhazin at all, which is what I have done.  It's up to you, but personally the benefit of keeping the Rhazin fusion available at all times has allowed me to continually fuse the epics for this over and over.  This has not only allowed me to get easy points for champion chase touraments, but also allowed me to fill out the various epic faction guardians quite easily.  I then use any remaining as free 4 star chickens.

The downside is that you halt all progress in the missions and will never get Rhazin through this fusion path, but you can get him through normal summoning.  It would depend on how badly you need him for your account, though in fairness he is a very good champion

Just putting that out there as a consideration, because once you do fuse him you will lose the ability to fuse the epics ever again

Hmm...

I actually think this might be a good temporary strategy for new players until they get to the Rhazin mission.  Leaning towards, yes good strategy  :)

But, once at the mission claim and move on!!! So fuse the 4 epics and lock in reserve vault unless you absolutely plan on using rhazin. But like most champs he got power creeped, imo.