Just for fun, I tried a battle against harpies, splitting them into 6 stacks with 8 spell power. The losses were 6 fairies. So, it's not that important for further progress how many groups the harpies are divided into.
In any case, the first two days are even a bit easier – I only lost 8 fairies, and Luna reached the next Pandora.
P.S.
I highly doubt that top mapmakers will create maps designed exclusively for HD+.