For anyone wondering about hydras in 2.4, here is my experience on it so far. I’ve been rolling a FO/hydra sorc since the start of ladder for mf’ing and bossing.
Pros
-they finally made hydra a very viable level 30 skill. The cooldown is removed and you can now summon up to 6 groups of hydras as fast as you can cast. That is 18 hydras you can have up at a time. My hydras currently do about 1.1k each with maxed hydra and FM. 18 hydras x 1.1k is almost 20k damage total every time they spit bolts which can melt stuff pretty fast. You can probably get up to 2k+ with full synergies and +skill items.
-hydras are good for scouting out or luring out targets outside of your screen. just cast them ahead of you and see if they start attacking anything. One trick I use for bosses is to spawn hydras and then teleport to another room. The hydras will keep hitting them until they despawn. rinse and repeat until dead.
Cons
-hydras like most of the other fire skills require good timing and/or good set up. In order to get the full damage output you have cast all 6 groups which can become quite tedious and requires good fast cast. This is one thing I hate as you have to count how many times you cast or you will interrupt your hydra attacks if you keep casting more than 6(i wish Blizzard would add a hydra counter or something to the skill). This also requires much more effort than say a single click meteor or firewall spell. I usually rotate hydras with my FO while it is on cooldown. FO->3xhydra->FO->3xhydra to get my 6 groups up.
-although hydras can put out some good damage they are not a true AOE spell. Each individual bolt will only hit one target. Which means that they are not great in areas with high density like cows. You will need a secondary spell like blizzard/lightning/FO/high lvl static to support them. However, for single target mobs like Mephisto they can melt him pretty fast
-You can’t control the target the hydras are aiming at. It seems like they will always aim for the nearest target. This is annoying when you are aiming to kill a boss or unique like Andariel but your hydras end up targeting a fire immune mob instead. You have to either spread out your hydras or lure the bosses right on top of the hydras so they will attack it instead.
Conclusion
Blizzard finally gave Hydras the love they deserved. It is a very good spell now and I wouldn’ t be suprised if it becomes the new cold/fire meta. But for me it requires too much effort. I like my single click spells like meteor, blizzard and firewall. Although Hydras are fun I’ll probably end up switching back to FO/Meteor or FO/Firewall until I get an Infiinty then I’ll just go full lightning.