The pyrphydra, on a full attack at +17 per attack (+2 of charging) hits on an 11 or higher vs normal AC (9 or higher if charging). 12 attacks at 2d8+6 per attack (15 damage each, on average) is 180 damage. A 45% hit rate deals 81 damage. On a charge, it has a 55% hit rate dealing 99 damage.
IN order to hit the hydra, the monk has to move through threatened squares, provoking an AoO from all of the Hydra's heads.
I do not think the hydra belongs on the list of things the monk can face in melee.
Yeah, if you hit it hard enough, you can deal 2d6+2+5d6 damage, average of 26.5 points, but that's 20.5% of the hydra's HP, and the other 3 members of the standard party to deal with it in one round before the monk gets ripped to shreds. This means they'll be taking down 26.5% of the hydra's HP each, assuming everyone contributes equally, which would imply that the monk is actually behind the damage curve.
As for the Ice Devil, it flies, teleports, can cast Ice Storm and Ice Wall, and has Persistent Image, all as at-will abilities. How is the monk engaging it?