It all depends what bosses you do and don't like really.
I chose Koloktos first because I knew that I would lose a few hearts in that battle. The next bossed that he would throw at me were ones were I could easily get hearts (like Scaldera and The Imprisoned). I would advise you not to choose these bosses first.
Umm...as for the Imprisoned, in the earlier stages of the battles against it, I would advise that you try this:
Try to stay in front of it most of the time, target it and then use the Skyward Strike. It worked for me and meant that I didn't really have to go near it!!
I know this challenge can be a little tricky, but believe me, you can do it! After all, you only need to fight 8 bosses to get the shield (which will
really benefit you).
Hope this helps! Good luck!!!