The most difficult crystal to reach in spider cave is most deffinately
the one in the water near the dark eco pool. The only way to get to it
is to make a role jump by pressing L1/R1 + X in your role. You have to
time it perfectly to make it but it's absolutely doable after some
practice. If that's the only power cell you can't get: don't worry.
There is a total of 101 power cells to collect in the game and for the
alternate ending you only need 100.