Hi, I'm new here at EoFF, but i thought I'd give riddling a shot.

So for my guess I'd have to say HP/MP-materia (you know, the one that swaps your HP for your MP)

The high shall be made low - the high amount of hp becomes the low amount of mp
The low shall be made high - same as above except the opposite
Useful for pushing some limits
Even against the humblest foe -it makes battles hard even one weak enemies

I know it's my first but still... you never know... perhaps its right