Help Building a Boss!

[GreyBlack]

So I'm currently building an adventure path for my players. There are 4 guys in my group (Barbarian, Summoner, Warpriest, Inquisitor), and, for the final boss, I'm thinking that I'm going to throw a summoner their way. I expect them to fight this guy around level 5, which leads me to the question of how strong this boss should be for a balanced fight? I was thinking of making the boss ~level 8, but that feels a bit too weak for a group of 4. At the same time, level 9 feels way too strong, because Summon Monster V is when things start to get ridiculous. So, my question is, what level NPC should you throw at your PCs for a balanced encounter at level 5?

[Callin]

I would make him 5th. Give him Max hp and add traps or other deadly factors into the CR to up it.


If you are worried about him being too tough at higher level. If not then disregard.

[Abd al-Azrad]

The tyranny of action economy would indicate that your best bet would be not to build your boss to be more powerful by giving him resilient minions. Already being a summoner, this would likely come naturally to this dude.

You've got your heavy-hitter in the Eidolon. You could keep the Eidolon as a reserve force (brought to bear by the Summon Eidolon spell, or whatever its name is) and begin the fight with the highest-level Summon Monster you can manage (or Augmented Superior Summons of something one level below your highest-level, for maximum battlefield control).

What you need now are tanks and action-denial. Swarms are great for this with their natural damage resistances and nauseating attacks. Elementals or constructs can be useful for crit- or spell-immunity, if your party has a particularly effective form of attack. You should select lower-CR but resource-depleting foes for these purposes; make your party either burn through them the hard way, or solve the combat through clever tactics.