story opinions wanted

i'm currently hosting a d&d campaign (with very heavily modified rules) for some first-time players, only 2 are familiar with d&d so i just really mad it so the dice decides most things. i just moderate what they want. but i don't know whether i should keep the story i was originally thinking (necromancer takes over the kingdom and they have to train up to defeat him) but A. i'm running out of encounter ideas and have just been doing the same stuff over and over again and B. it is very annoying listening to one person constantly asking to shoot other people with their bow. i want to make them fight a false hydra because A. they will finally have to think about things and B. it makes for a much more fun final battle, but what would you recommend for me to do i'm open to ideas

A. I don’t think this issue is because they're fighting a necromancer, how long has the campaign been running? What enemies are they fighting, how varied are the environments and how close are they to achieving their goals?

B. What are you talking about? This is just regular D&D combat (I cast this spell, I attack this guy), what’s the other person doing here?

A1. How exactly will they have things to think about. Why don’t they have things to think about already? A false Hydra is a very hard creature to run because it isn’t actually made for D&D, it’s a micro fiction that uses the D&D setting instead of the actual system.

B1. Why would it be more fun than a necromancer? A BBEG aren’t inherently more or less fun than one another, why would a random False Hydra fight be more satisfying than fighting the necromancer they have been going against?

A. the campaign has been running for about 3 weeks but they are losing interest quickly. the enmies are typical undead and skeleton undead. the environments are a forest similar to a pine forest and a nearby town based around medieval times

B. the one person is a player and because they are bored they want combat so they are trying to start combat with other players

A1. i haven't been able to think of any puzzles for my players since i'm going with an open world and the most intense thinking they have done is what form of training they have done

B1. well i don't have any experience with necromancers (or any BBEG) since i'm a first time DM but i've been playing for about a year and a half now but wanted to try DMing and i think the thrill of defeating such a powerful beast on their first campaign might get them into proper D&D

bonus info i'm open to other ideas for encounters i could do

3 weeks is really not a long time, you should mix in more encounters that aren’t undead. What roleplay have you guys been doing in the meantime? If your going with open world, you should have a few distinct locations and people that they could choose, it’s a lot better than making stuff up on the spot.

The problem with defeating the false Hydra is that just fighting it isn’t the point. The threat it poses is that it’s extremely hard to detect and forces the party to be perceptive of their surrounding, after they discover it, they will just beat it to death like normal. If you want to go for killing a powerful beast, maybe have them fight a dragon of some kind? It really depends on their level. Even still, you need to build up whatever monster you want as a credible threat and something to work towards, not just meet and kill.

the roleplay done so far has been relieving tension with a character exploring the town healing an injured dragon (but was unsuccessful).and i'm currently making them investigate someone who seems suspicious they are only level 3 though which is why i've been doing filler roleplay stuff because level 3's can't go up against a BBEG but i've also been trying to train up their dragons (which were from the clutch of eggs from the big dragon) but power scaling so it stays balanced

Level 3s can definitely go against a villain of some kind, you could have multiple BBEGs and antagonists for them to go get at, there’s no reason why they gotta do nothing for a long stretch of time.

Level 3s aren't usually supposed to go battle a BBEG. They're supposed to do some local investigation. And as ClassicCharismaCaster said, 3 weeks isn't exactly a long time.

I do agree that fighting just a bunch of zombies is a bit boring, since it's essentially just a hack and slash. Maybe give them a combat where the monster's point isn't actually trying to kill the characters. For example, if they're riding on horseback, maybe a predator attacks their camp while they're resting trying to kill the horses, because it's hungry. Give the monsters some motivation other than "we're evil, we kill things".

For the one player who's bored and tries to start PvP, I'd suggest a session zero. I borrowed my method from Robert Hartley GM, which is that PvP is allowed if both players want, but if player 1 doesn't want, then nothing that player 2 does will ever hit or deal any damage. Even if they're trying to cast an AoE spell at point-blank range, it'll do nothing. While we're in an actual combat against actual enemies, then AoE hits, but at that point they usually want to avoid hitting their friends.