Any tips on fighting a primal hydra?

Per d20pfsrd a straight hydra is CR 4 so my APL 6 party should have been fine but with 28 AC, DR, blur, and 9 points of fast heal per round I had way too hard of a time with my first Primal Hydra fight. I assume I am missing something but can't figure it out.

- As far as I can tell there is no way to sever/sunder the heads in the game interface.
- The only attack I have found that isn't affected by DR is an acid attack.
- I have found no way to lessen/prevent the fast healing

one of the elements should stop fast healing. don't remember what it is for hydra. fire/cold/acid/negative energy/cold iron. electricity is as useless as ever. everything and their dog is resistant to it, don't think it stops regen on anything either. considering it's fey it will likely be either cold iron or negative energy

During the fight I tried fire, acid, and cold iron but will try cold and negative energy next time.

Do you know if there is a minimum amount of damage required (e.g. the 5 points to stop head regen in base rules)?

If so fire and cold iron might still do it since with DR I never got more than a couple of points scored per round.

any damage of the element will stop regen for the next round. if the thing is on the ground it will kill it. if creature has DR to the element it won't stop regen as a rule of thumb

oh also blur and concealment is why every caster ever should have glitterdust ready :P or it is pain. this game has lots or such creatures.

Always use "protection against poison".

Its a rather low-level spell, i use the group one. Its a must against a lot of monsters.

depends if mobs can hit your frontline at all or not. if they need 20s you might as well skip it to save time.

Doesn´t matter. Otherwise you have to deal with all the attribute loss and such things. And in early and mid game, this is a bit harder to manage. Later on you have the counter spells en mass, but at the beginning, it can make a lot of harm.

Edit: Even in late game, always when i see "poison", or some beast-type, i take in the anti-poison spell out of pure reflex. It helped me a lot to survive this kind of encounters.

primal hydras die in 2 rounds give or take. I agree that communal delay poison is good if there is more than 1 of those due to breath weapon. that 1 of them might have time to use. physical attacks of regular stuff doesn't matter too much once your front-line has 50+ AC