fix several monster difficulty ratings

fix several monster difficulty ratings

Fix most of the things pointed out by #wizmondiff.

Weakening of placeholder 'elf' is due to recent removal of M2_STRONG
for it as part of the "orc strongmonst" changes.

I assume that the discrepancies for multiple quest leaders came about
as part of the change that allows killing the leader as an alternate
way to gain access to the lower levels of the quest, but didn't check.

I don't know what's up with 'piranha' but just changed it to match
generated value.

'{freezing,flaming,shocking} sphere' still show up as discrepancies
with hardcoded (mons[].difficulty) value higher than generated value.
They got harder when their explosion was beefed up, so the formula to
calculate difficulty ought to be updated to account for that.