give serpent of hell dis a reaching attack

this should make it a bit more dangerous (it seems to be the weakest of
the four by a good amount), and also fairly distinctive from the others
also tweak serpent of hell hp

coc and geh variants are a bit on the strong side - i don't think that
the serpent of hell is overall too dangerous, but i'd like to balance
out the level of danger between the branches a little better