There is only one head that can be provoked in Hydra, being Decay.  Is it possible that head wasn't in the starting rotation?

Provoke is essential for Hdyra, but only on Head of Decay, who is not part of the initial 4 Heads this rotation.  However, Decay will pop up as you kill heads and they respawn...

490 Acc should be enough to reliably land Provoke on any level of Hydra with his A2.  The Provoke Gear needs zero Accuracy to land, but only lands at the rate of 30%.

Thanks for the replies, I thought with the set and his A2 guaranteed it would work. Didn't really understand that rotation had an effect.

So do I need any accuracy at all? If it's 100% when booked can I make him more Tanky in exchange for some of the 490 accuracy I have on him.

100% chance does not guarantee that it lands, just that you have a 100% chance of trying to land it.  You still have to win the Acc/Res roll with any skill unless the skill says something like "cannot be blocked or resisted".

For Khoronar specifically, without books, his A2 would only give you a 75% chance to attempt to land the debuff.  

Gear Sets do not need any Accuracy to land. 

As far as how much Acc is needed, that depends on what level of Hydra you are fighting.  You can get into the Stat details here: https://hellhades.com/raid/stages-tool/ 

Thank you very much, that link will be very useful.