I'm not TOO sure, but I think she has about 2500HP in normal and 5000 in NG+. Since Rainicorn's attack always does 25 damage, it means 100 hits (200 NG+).
The problem is that if the stars hit ANYWHERE on Rainicorn (even the part behind Finn and Jake) you'll still take damage. You can't use items either.
Just keep trying, you'll get the hang of it eventually. In NG+ I've beaten her 3 or 4 times and I still can't do it without losing less than 3 full hearts. --- 3DS Friend Code: 0001-3567-2030 passid.ca/zai