Help designing a villain

[Kelban Alenark]

so im designing an encounter for my party that will be the cap to a mission they are currently on for the town they live in. They where initially told that their had been sightings of a forest drake in an area near a trade route and were asked to kill it before it started attacking caravans. They get their and fin out its a very young green dragon. It parleyed with them and explained it didn't want to fight anyone and that it just wanted help protecting he remains of its mother. Plot reasons that don't need to be explained its mom was an ancient wyrm who was killed by a big bad they will fight later in the campaign. Well a cult of Urgathoa has found the mothers tomb and is attempting to conduct a ritual which will raise the dragons body as a zombie. both the mother and the child are LN and worshipers of Irori so the party has agreed to help.

Now here's the deal i want it to be an epic fight with a necromancy based Occultist with a bunch of mutated trobblin minions. I intend for him to be the apprentice of the main necromancer conducting the ritual and the party is going to interupt the preparations while the Head necromancer is away. I HAVE NO IDEA how to build a good occultist to begin with let alone for this situation, i just really like the idea and ascetic behind them, and i also don't know what kind of necromancery class would work well as a master for an occultist.

So how would you build this encounter or at least what are some suggestions that could help me build it. The party is 6 level 4 pcs with a very young green dragon ally.

[My Self]

The best necromancers are clueless forumers.

That aside, the best necromancer player classes are Oracle, Cleric, and Wizard, roughly in that order. All of the above would make excellent bosses if paired with an appropriate set of mid to low-level minions, and maybe one or two tougher bodyguard types.

As for your battle...

1x CR 6 Necromancy-focused 6th level Occultist
1x CR 6 beatstick-type monster
10x CR 1 3rd level Warriors, refluffed to be mutated minions

This is nearly a CR+4 fight, so the PCs should come in with all their daily abilities and at more or less full HP. The battleground should not be heavily biased in favor of the Occultist, although you should adjust as needed if the PCs are unusually competent, incompetent, lucky, or unlucky. The Occultist and beatstick ally should start together, behind some sort of cover, so they aren't shot down immediately. Make sure that PCs are given comparable defenses. Perhaps 5 or so of the 3rd level warriors should be deployed at the start, with the other 5 hanging back with bows or ready to jump in and surprise the PCs. Start with 4, and add more if they steamroll, or subtract some (to take care of the wounded) if the PCs are getting beaten about. They are probably not going to be terribly damaging, but their job is to soak up damage. Be 100% sure they aren't bunched up and don't get collectively destroyed by Sleep, Grease, Glitterdust, or anything similar. For this purpose, add some light cover to the terrain. The Occultist and the beatstick monster should be a tag team - the Occultist buffs the both of them, then sends the monster out to beat up on the players while he does crowd control or ranged damage. Be sure to avoid using save-or-lose spells such as Glitterdust, since players hate to have their agency taken away from them. Depending on if the Occultist is the cowardly type or not, you might have him pop Vanish once the PCs knock down his allies.

[Lady-J]

i would only start out with maybe 4 of the warriors and have one or 2 "back up" waves ready to go if they party looks like their going through to easy however starting off with less can mean you can easy up on them and not send in the "back up" waves if things are going poorly i would actually have the necromancer be a cleric and use a summoner for the other cr6 creature

[Pizza Lord]

Seems a real shame to have that Ancient wyrm corpse there and not have the bad guy try and animate it. A typical Ancient green dragon would have 23 HD. That's puts it beyond the power of animate dead to turn into a skeleton though. Of course, there's no reason it can't have been a 20 HD green dragon, just below a typical ancient wyrm in power. Have to adjust a few stats.

Alternatively, if there's enough flesh on the mother dragon, he can raise it as a zombie, which typicall would be 23 HD but would gain+6 HD because of gargantuan (with a Colossal slam).

There would likely need to be a desecrate spell in place to allow the animate dead to create 4x the HD of undead for caster level rather than the normal 2x. So if the bad guy is 6th-level you could get 24 HD or if 7th-level up to 28 HD (still not enough to animate a 23 HD zombie with the +6 HD bonus, unless you make the dragon about 20 HD.)

Assuming he can access haste or remove paralysis he can make it a Fast Zombie, which can not only make a full attack but also gets an extra slam when it does. Of course, that's like a CR 9 beast and would likely crush the party, so a normal zombie green dragon might suit the situation a bit better.. Even though that's really powerful, the partial action should really tone things down.

An important thing to note is that the bad guy probably can't control that much HD of undead, even in a desecrate, but it could just do it and fall back a bit and let the PCs deal with the uncontrolled skeleton/zombie dragon. Although in a pinch, a command undead, if available, should allow it basic control of the beast with no saving throw (since it's unintelligent). Of course, well prepared PCs could try the same, not sure how the younger green dragon would feel about that.

Of course, the best part is that the young dragon now has its mother's remains animated and attacking the party and it's probably yelling not to destroy her corpse. That really makes the scene more exciting and memorable. Even if they beat the badguys or drive them off, how they treated its mother's remains could be important. If somehow the PCs disrupt the ritual or spell that animates the dragon, at least they'll have earned one hellaciously large onyx gemstones.