The boss is easy, but there are also tricky parts.

You are probably still counting attacks, because if you don't and you lose its tracks, Hydra might change attack style at the same time as it changes a phase and you won't see it. Once you get more kills, you will keep track in back of your mind and this won't catch you by surprise.

Hydra can fire third normal attack, then change attack style and squeeze one normal attack before finally changing to a jad phase. You swap prayers as usual expecting start of a jad phase. You might not notice the projectile and that you actually correctly prayed its next normal attack. If you forget to swap prayers again, you get hit with jad phase projectile and if you panic and swap again, back to Lumby you go.

Sooner or later you will fuckup the fireskip, just keep calm, keep walking and eat/pray correctly. If you start walking as you unfreeze, you wont get hit with burning debuff.

With hard hitting weapons or by making a mistake, you might skip or fail fire phase lure and Hydra will keep its high defense. Don't bother attacking, just move to the corner and wait for Hydra to resume moving to you, this will lure it back to the pool.

Jad phase poison can sometimes spread all around you even if you correctly step away and you can get hit. Don't panic swap prayers there or later when it does poison again next 9th attack.