We can see Bijuu Power cannot be judge by count of tails, 9 tail can create tail beast bomb equally 2,3,4,5,6,7 tail beast bomb fusion, 2+3+4+5+6+7=27... and 9tails can defeat them.
if we believe
1+2+3+4+5+6+7+8+9 = 10 tails
>> then we can open mind that
1+2+3+4+5+6+7+8=9 tails
so we can use...