Introduction

 The weather in Barova hasn't been much of an issue that I can recall. We established that it was always gloomy and never brighter than twilight, but there haven't been weather events I can recall. This would be a good moment for one. If we had a party that was really into ranged combat, I would suggest playing around with penalties and such. Instead I just recommend using the onset of an unusually severe storm as an ominous bit of mood.
 As far as the actual baddies, I envision Leonidas as a large lion with nightmarish features. He prowls around a clearing in a dense forest. The Initiates of the Four Winds -- his druidic disciples, are in a square or diamond pattern each 50' apart. The idea with the geometry is to make two targets in a line fairly easy to set up, but three unlikely (and impossible from inside the formation.) The "Four Winds" thing lets you mentally put NWES in place. Even if cardinal directions aren't your strong suit, it may be that other players can better navigate in your mind with directions like that. If it doesn't work out, there is no downside, since the four winds make a perfectly good theme for a small druid circle.

Tactics

 The Disciples (SPELL DC 14) say hello with a Call Lightning spell, so their main jam is to maintain concentration and keep dishing out those 3d10 zaps. After round one, they have their main action to Produce Flame or Cure Wounds as needed.* They may also chip in with a Dispel Magic if needed. Disciples who lose concentration on their Call Lightning but survive being attacked may follow up with a Hold Person. Their role is to generate steady offense and intervene where dramatically appropriate. For example, they could all attempt Hold Person on round one, but that's a big move. Each casting could shut down a troublemaker or get a PC to hold still for a melee thrashing. Their best plays should be made selectively, and it may not be a big deal if you forget one or more of them. From the start though, they should process as an easy batch of Call Lightning saves.
 Leonidas (SPELL DC 17) is more about the rarr. If no adventurers are near, he should throw Lightning Bolt as you requested -- possibly even at an elevated spell slot. Alternatively, he may multiattack as a claw/claw thing. If both are effective, then his foe is pinned (i.e. grappled.) Either way, he is also eligible to make a bite attack as a bonus action. The alternative to that is a a powerful roar (15' cube) that does thunder damage (2d8) and pushes people away (10') -- (i.e. the Thunderwave spell) as a bonus action. Also note that his movement is sufficient to jump over a Wall of Fire. As a druid corrupted by bonding with a decidedly unnatural land, he could cast spells in beast form and know pretty much any nasty twisted spell of 5th level or less.

*I miswrote. The lightning strikes require an action, so Healing Word is really the only druidy thing a disciple could accomplish while working the storm. Also, damage is 4d10 if the lightning is drawn from a storm that existed before the spell was cast. You're going to want to be fairly spread out with this stuff because it could feel like four bolts from the blue. Perhaps allow for (and hint at) the idea that the storm itself is magical and could be dispelled. Either way, the disciples should only be doing full Cure Wounds or making their Produce Flame attacks when they aren't calling lightning.

Reinforcements

 Perhaps this executes so strongly that you're pulling punches instead of trying to push back. However, if you are trying to push back, druids are extremely versatile. It is possible to explain the arrival of literally any beast. Also, I'm thinking the drama should be that, when Leonidas hits 0hp, he becomes a human again, with ~50hp. If he is not put down before his turn, then he can Wild Shape into a dinosaur or a giant ape. Also, he could take an action to Polymorph a disciple into one of those creatures (though his concentration would not likely hold long. Alternatively, dinosaurs could be conjured by scroll or other one-shot device. If you want to go another way and still have druid-sensible reinforcements, conside earth elementals. Brick is the stat block for that, and druids conjure elementals at least as well as wizards.
 All this stuff drops in pretty smoothly. Perhaps Leonidas has to go a round without paw attacks to conjure something, or perhaps a disciple uses his action to read a scroll. If you're planning on allowing rest after (or 7 in 8 chance of rest after,) I would keep pushing until it feels like our bag of tricks is running low. You want to hear Tony say "what spells do I have left?" You might even get me to say that. I certainly think it would be memorable if, when I played a dinosaur, you countered with an equal beast. I would worry less about dropping us (which would be trivial for me if you chose to concentrate all those Call Lightning shots raining down,) and more about creating an "on the ropes" feeling. I'm suggesting you put on the table enough to take us down, and then pile more on top; but I'm also suggesting it will perform perfectly if it just gives us a thorough workout. Then again, if Leonidas is victorious, does that mean I won Curse of Strahd?