I will say that HP increases after 7th or 8th stars should be more scaled to the amount of gems and experience needed to get to the next levels. A "progressive" HP increase proportional to the amount of experience.

But I agree the 15th star is unachievable and this whole progression should be made so that it can be achievable.

I am not going to propose a solution, but I am sure an acceptable workable solution can be derived fairly easily.