I'm a fairly new DM, only started in January and I am currently running a successful game with a group of players. I'm looking to start a new campaign with different players and have an idea of what the BBEG is going to be, I think the game will only go until level 5 or so, as I don't know how committed in the long term the players may be. Currently I am successfully balancing encounters of my own creation to the players, giving them challenging epic fights every so often, with a close fear of TPK but they pull through, and other fights that act as attrition style fights that slowly wear them down over a course of encounters. But these are fairly standard slug fests, and I want to shake that up a bit.

In this new campaign, I want to throw a Hydra as the final encounter as a lvl 5 boss fight, and I think the fight can go one of two ways. The players learn how to defeat it, ie deal 25+ damage then blast it with fire to 'stump' the head as they have learned how to deal with it; or the players will just keep going for damage blindly until it runs out of HP. The latter of course being a much more deadly fight, as killing a head and not blasting it with fire will grow 2 more!

My question though, and specific example aside as there are monsters that might be vulnerable to a certain damage type and knowing that upfront makes it easier, what advice do my more experienced DMs here have on allowing players to learn of weaknesses or tactics that will make the fight easier, or achievable at their level? Do you have knowledge supplied to them prior to the fight from a sage or previous failed adventuring party they need to seek out? or rely on the first couple of rounds to drop in the hints of what is making an impact so they learn on the fly?

Just to follow up, I was thinking of one of two options;

A side quest in the lead up to hunting the Hydra takes them on a side quest that finds the last survivor of an adventuring party that attempted to kill it, and he found out too late how to destroy it, but is willing to part with some tips

or if the players do not go on that side quest, they come across a baby hydra which isn't too challenging, and that acts as a practice fight where they can learn about the monster during combat. Killing it too quick, or just slugging away will show them nothing, but the opportunity is there for them to try without the Deadly+ encounter rating of the final fight.

I'm of the thought train that information should be available to the players should they wish to look for it, and that's part of the adventure. Do you guys disagree and think it should be kept back?

Allow the players to discover things about their opponent ahead of time, if you want them to know it. The info could be learned from a retired adventurer whose party was killed by the hydra (and it ate his leg) but he saw that when his sorcerer hit it with a fireball, the destroyed head didn't regrow. The adventurer hates talking about his adventuring days because of his grief, but if the PCs perform a service for him (perhaps finding him a new mechanical leg, or protecting the village where he now works as a butcher) then he'll share this information with them.

oh I really like that, gives me another side quest to throw in there as well. I want to allow the players to find out that it will be a tough fight regardless, but the idea of sending them off to help the NPC before he imparts information is good, maybe adding a timed element to it so he's been captured and they need to save him before he gets killed, or is up for murder and they have to prove his innocence, then not completing that means the information is lost.

Thanks Sanvael, this is why I like bouncing ideas off of people!

Yeah, giving the players ingame hints and clues is always a good thing and can be a fun sidequest in and of itself. Maybe they hear a rumour that the Hydra can't be killed by normal means and so they figure out that they must find the right way to do it.  Perhaps this particular hydra has fought adventurers before which means it has on neck that us just a burnt stump? There are many ways to do it. Good luck with the game!

Yea that's a good option, if the players don't find out the information prior to the fight (of which I'm lining up 2 side quests they can choose from, both reveal the same info as reward), then have a hint there of a cauterised stump. That way, the info they get from the quest is that removing a head will regrow 2 more if they can't find a way to stop it, but they don't know about using fire on the wound; or they get to the fight and the regrowth of 2 heads is a surprise, but they get the hint of using fire on a wounded neck.

This is good food for thought for other boss fights, as this is an extra dimension to the encounter. I just hope my new players will act a bit more intelligently than "I run in and swing my big sword".

I might allow a Ranger or Inquisitive Rogue to make some checks, a mix of what they know and what they're seeing.