Help with boss design

[Shiki-pon]

So, I've hit a bit of a roadblock with the game I'm currently running... There's a certain BBEG coming up, but for the life of me I can't think of a build to represent what I'd like him to be capable of.

Ideally this guy should be a spellcaster that can handle himself in combat, preferably divine... But my issue comes from the fact that his background would fit an arcane (not a magus though)/divine spellcaster better. I'm planning for him to be around CR 12-13, and as stated, capable in close combat and a spellcaster.

If anyone has any ideas on how this could be made to work, I'm open to any suggestions.

Also, if someone has any understanding on how exactly the Shadow Lord template works (I'd also like for the BBEG in question to have the template, although it's not strictly necessary, would just fit the theme I'm going for) then an explanation on the mechanics of the Incorporeal Step abiility (especially its interaction with weapons and armor) would be greatly appreciated.

Thanks in advance for any help you can offer.

[Glimbur]

Arcane-ish background but divine caster sounds like an Archivist (Heroes of Horror) to me. That doesn't help a lot with being capable in close combat, unless you cast the standard clericzilla spells. Alternately, a cleric has 3/4 BAB, heavy armor prof, and divine spells. We may be overthinking things.

How have bosses worked in your games so far? I have trouble using just one tough guy because the party gets a bunch of actions to their one. Disabling/dazing spells are an answer but they can reduce player fun. You could consider giving him multiple actions because reasons, minions, or serious shenanigans with the party having trouble finding/affecting him. That's my standard boss rant.

[Shiki-pon]

The archivist sounds neat, but since I don't have access to it... I mainly use the online pathfinder database, but depending on how it works, it might at least cover for the spellcasting part.

Usually we just give the bosses an extra action or two per round and call it a day. It's worked out pretty well so far, against 4 PCs. I've also been toying around with the idea of player action's triggering action's from the boss (obviously a limited number per turn), but I'll still need to test that idea further. Also, I've used the D&D 4th ed boss monster conversion (more hp, better defenses etc) with some advice from a GMing blog thrown in (fight happens in phases, boss does different things each phase and so on).
But yeah, I have noted the problem with the player's outnumbering the boss, it's just that the situation I'm planning doesn't really allow for minions to be around. Also, most minions just plain can't touch the PCs...

[Engine]

Animate Dead, Create Undead, Planar Ally and Summon Monster are your friend. The minions these spells create aren't there to kill the party, but to keep it busy. Every action the party waste on the minions is an action the party doesn't use on the BBEG. and some of the minions you could get with these spells could still do some damage to the party if left unchecked - and maybe buffed a bit.
For the BBEG have you thought about the Shaman (http://www.d20pfsrd.com/classes/hybrid-classes/shaman)? It's a hybrid between Oracle (divine caster) and Witch (arcane caster) and with the Battle spirit seems to be capable of going in melee.

[Shiki-pon]

Unfortunately Summon Monster is the only summoning applicable in this case, since the PCs are most likely going to catch the BBEG off guard, so the situation forbids the use of spells that require corpses or a 10 minute casting time. Then again, it would fit the guy's undead theme to use Skeletal Summoner and summon a group of skeletal creatures...
And shaman isn't quite what I'm looking for, though it might fit something else that I'm planning to include later. Just for clarification, the melee idea was that the guy himself could at least handle himself in close combat, so having another type of minion doing that for me doesn't quite fit the picture. Still, thanks for the suggestion.

[Engine]

A Shaman is a 3\4 BAB class, with medium armour proficiency and D8 HD. Battle spirit gives you some nice combat buffs: Enlarge Person, Magic Vestment, Righteous Might. It seems to me that an enlarged Shaman with Righteous Might could be quite good in combat.
Animate Dead and Create Greater Undead aren't meant to cast on the fly; a BBEG casts these spells in advance. Even if the party catches the BBEG off guard, why the BBEG should willingly separate herself from her undead guard? The same could be true for Planar Ally: the BBEG has struck an agreement with a Glabrezu that now serves as her bodyguard. So even if the BBEG has not the time to muster all her forces, she still have her personal guard at hand. Unless the party has a plan so the BBEG will be completely alone at the time of the attack, I don't see why the BBEG couldn't have minions that she summoned days, even weeks or months before the attack and that now follow her.

[Shiki-pon]

For some reason I can't take the BBEG seriously anymore if he swells to 4 times his original size just to fight. No offense meant, but if I want a giant monster, I'll just take one.
And yes, I know that the BBEG can cast the minion spells in advance, just not in this case. This is because the BBEG in question has been sealed away for a length of time, and the PCs confront him just as he fully comes back. There are some preceding encounters meant to mitigate the party's effectiveness against this guy, as well as to serve as a distraction from the unsealing itself. Since he basically just wakes up, he most likely doesn't do so with hordes of minions at his side. Admittedly, I didn't elaborate on the circumstances, so my bad.

As for the shaman, the spirit animal is apparently a different type of familiar (again, problems, whoever put the BBEG in a can most likely wouldn't have the courtesy of sealing one of the sources of his power in the same can as the BBEG himself), so that's a negative... But for clarification, he's supposed to fit into the basic Evil Sorcerous (not meaning the class) Overlord archetype, and I'd at least think that him being a threat in melee would add to his image. The divine part isn't that mandatory, it's just that he discovered necromancy through a (kinda-sorta) god. But since there doesn't seem to be much of a difference between arcane and divine casters besides what you can wear and still be able to cast spells, I guess that whether he's divine or arcane doesn't really matter. The reason why I would've wanted both is to get both the feel of clerical spells and the variety of arcane spells.
Mind you, the original concept was built using the Sacred Thaumaturge (http://mcarchetype.wikispaces.com/Sacred+Thaumaturge) multiclass archetype. The main problem with that one was how sucky he got in combat without first buffing himself for several rounds.

[Novawurmson]

The three important tools for a great bossfight: Minions, setting, and defenses.

You boss needs minions/lieutenants/allies, otherwise you players will crush it with sheer numbers. If possible, try to make sure your minions round out the boss' weaknesses - if your boss is all melee, have some ranged minions. If you boss is all spellcasting, have some mundane minions. If you boss is an undead, give it some minions that aren't harmed by positive energy, and so forth.

Setting can make or break a boss fight. Make sure the players are fighting for something - if the boss is smashing down the gates of the city they love dearly, awesome. If the boss is minutes away from the ancient portal that will summon a dark god to destroy the world and the players are in a race to stop him, awesome. If the players are assassinating a mob boss and need to make sure no witnesses escape to warn the rest of the gang, awesome. Make sure there's always some goal beyond "kill this guy." Moral quandaries also make for memorable fights.

As a whole, players tend to optimized offense over defense. This is good for the players - the faster they kill things, the less likely an unlucky roll will kill the character they've spent hours (or days, or years) developing. To make a boss fight interesting, you don't want to optimize damage. The more you optimize damage, the more likely you'll kill off a player, and the more likely it is that your players will just one-shot it. However, if you have a boss with high defenses (HP, AC, miss chance, counters or other immediate-action blocks, reach, resistances and immunities, etc.) and moderate damage, it'll be able to take a few hits before going down. Your players won't be able to count on "I full attack; it's dead" and will actually need to do some strategic planning and decision-making.