RolandOfGilead said:
I refer you to the example listed in the MM of the red dragon skeleton. It has the fire subtype.. it doesnt say subtypes are removed, and therefore the dragon as listed has immunity to fire. its not so much a special quality as the creature's subtype

the bones used to be immune to fire, what they suddenly stopped when it got animated?
Ergo, the pyrohydra has immunity to fire.
Alright. The MM does state that immunities are SQ based - but I'll buy your explanation and argument for Type based immunity. Immune to Fire it is.
As for web, yes, thats decent, however the thing DOES have a bitching strength. and I expect its heads can reach out of the web, can they not? yes there are ways to defeat such a creature.. if you know what you are up against.. but remember: its a skeleton. players are likely to understimate it... especially if it only shows up after the party is engaging a few other skelle's.
It has a Str 23. Decent but not epic. It has to get its entire bulk clear of the web before it can charge.
At Str 23 that's a +6 modifier to the roll.
Web permits a creature that made its initial REF save to move 5 feet for every 5 full points by which is Strength check roll exceeds 10.
The average roll is a 16. The hydra skeleton will be able to move 5 feet per round. After it is exposed (partial cover for one round) then still stuck for 3-4 more rounds getting out while on the boundary of the web, that is enough time that it is exposed and mostly immovable. On a ride by spirited charge with heavy lance, that's an average damage of 3d8 per hit. with 4-5 rnds to act - assuming those attacks hit - that will probably destroy it. Throw in one critical hit in thie mix - it's over.
Yes - it can attack while in the web if it did no fail its save and someone is stupid enough to remain in range. You might even give it its reach I suppose (while that's a ruling supported by the RAW, its starting to push it given the spirit of the spell effect and the size of the webbed creature and the effect of full cover when in 20 feet in a web...)
But never mind all that. As it can't charge and it is getting hammered by a ride-by attacks - there will be no AOO so reach does enter into it It's a ride by, not a *ride-up* - and the lancers should be able to get in and out of range of the reach with their movement.
Keep it in the web long enough (4 rnds exposed should do it) and that's it's for the critter. One 2nd level spell - no damage taken by party.
Web is an extremely powerful spell. I throw it at my players often
The web works only if the terrain supports the web. (Solution: don't let terrain support it.)
It also will only work because the critter is mindless. If it is being actively controlled by Munen the Wonder Raven, Munen should direct it to the other side of the web, out of the way of the ride-by path. It can't get to the characters that way (most likely) but it won't be summarily destroyed and the players will have to consume more resources to destroy it.