Hmm...

[omegatiger121]

I'm running a game in which the party will have an average level around 7.5 (one PC nearly to 7; the other nearly to 8) before adding in the higher-level PC's dad, a slayer, tonight. The encounter will be a gravewalking Witch and two skeletons. I'd like the slayer to have a higher level than the PCs. What level would be appropriate for the slayer so as not to swing the balance too far towards the party?

[Cuup]

Is "the group" just the two PC's? If not, how big is the group?

[Kileanna]

The real issue is not to swing the balance towards the party but to do it towards the Slayer. If you want him to be high level you can do it, but my advice would be to find a different role for him.
Why don't you add more skeletons to the fight and have the slayer to keep them away from the PCs so they can focus on the real fight? Don't even play the Slayer's part, just have him there keeping minions at bay while the PCs do the real fight. That way they see the Slayer as useful bit he won't steal their spotlight.

[Heretek]

For starters you should drop XP and start doing ad-hoc while keeping everyone equal. That'll help things a lot.

Secondly I'll agree with the above in that you should probably have the Slayer slightly off screen fighting off a mob probably.

[omegatiger121]

The slayer will have to be involved in the encounter because I'm only expecting two returning players with the third player being out of town and I don't feel comfortable GMing games with less than three party members. Also, the level difference is because the higher-level character's player hasn't missed any sessions at all.

The backstory is one PCs mother, a magus, having killed the gravewalker's skeletons when they were living, as opposed to their current state of unlife. Thus, the witch and skeletons are, mainly, attacking the slayer's daughter, a magus—like her mother—using her mother's weapon. Considering the encounter's at that character's home, which is in the middle of a forest, it's rather hard to add another skeleton to the fold.

[Cuup]

You could easily add Skeletons of wolves, bears, and other woodland creatures. Just saying.

The problem here is that even if you just made the Slayer level 8 - one level higher than the highest level PC - in order to keep the encounter balanced, the level 6 PC is going to start to really feel left behind. This is made all the more tricky, because the NPC is a primary combat class, meaning that he's going to steal even more of the spotlight - if he had been a support class, like Cleric or Bard, his presence wouldn't be so overt.

My suggestion is to just make him level 7, matching the highest level PC, but max his HP, and give him a little bump in AC and saves. This will let you show him taking an impressive amount of hits, or avoiding more attacks, or resisting more effects than the other two, but without overshadowing them. Then, you can just build an encounter for an APL 7 group without needing to do too much fudging on the level 6's part.

[SmiloDan]

I also suggest adding more animal skeletons.

Wolf skeletons can trip, stag skeletons can disarm with their antlers, boar skeletons can bull rush if they charge, bear skeletons can grapple, stuff like that. Just replace Improved Initiative for Improved X (Disarm, Trip, Bull Rush, Grapple, etc.) for zest (ignore the pre-requisites--they're just skeletons, so they can be Bonus Feats).

EDIT:

Maybe make the Dad GMPC a high level Warrior, so he doesn't dominate the encounter with lots of fancy feats and sneak attacks. He would just be a stalwart warrior keeping the skeletons at bay while the PCs focus on the witch.

[Lady-J]

bump every one to level 8 and dont worry about it, just keep every one the same level in the future