I think there are basically two approaches you can take for Hydra. Option 1 is something like this: https://raidbro.com/masteries?v=v1&m=0000013pds0o6d0

The basic idea here is that Timely Intervention is one of the best masteries in the game - as long as your team actually takes damage and gets low in HP.

The other approach is something like this: https://raidbro.com/masteries?v=v1&m=0subs000001ntio

The idea on this one is that you're not in a situation where you are taking much damage, because of various reasons, and so instead you just want to focus on being able to deliver the max utility possible. And, while Warmaster seems odd, remember that it has a pile of good benefits - it activates even on weak hits, which means even if they have poison cloud, and if you have leech, it heals quite a bit. And it also means that she can contribute to freeing a devoured champion - it's not much, but every little bit counts.

Personally I use the former one on my Nightmare team, and the latter one on my Brutal team. But that's more just incidental - realistically my Nightmare team never gets anywhere close to dying.

I prefer Warmaster on my Hydra champs but I certainly don't think it's a necessity.  I'd agree with the poster above that it heavily depends on your particular lineup.  If survivability is an issue then Support / Defense is probably the better route, if it's not it's probably better to get that extra bit of damage.

Warmaster, off/support 

The damage from Warmaster is based on the enemies max hp, not from the stats of your champs. All of your champs will do the same damage from that particular source, if they are supporters or damage dealers doesn't matter for Warmaster. The damage from Warmaster itself can't be critical, you don't need any crit damage gear to benefit from this mastery.

Warmaster too strong. Hydra is so beefy it just smooths out runs something fierce.

^^^ This, the extra damage plus assuming you have leech that healing is huge.  Even if you also use "cheeky" immortal sets :)