Huh? At that point of the game, you could have beaten it easily with all the charging and HP to spare.
OK, here's my strategy to S Rank (you may not need to S Rank, but this will help you capture it with ease). Charge in the any of the 4 corners, wait till its first move out of effect (ring of ghastly fire), then circle it. It will spit out ice blocks, circle Froslass AND the ice block. Do it slowly and steady if you are not too confident. After the ice block comes the Will-O-Wisp. After a specific amount of time, it will jump. So with some good timing, you will be able to dodge and circle Frosslass.
In conclusion, just circle it when you see it's possible and don't push yourself too hard when the situation is tight.