Creating an Awesome Final Encounter

[Mister Loorg]

In two months, the group that I DM for will be splitting apart due to real-life changes. I've told them that I'll create a final scenario for their characters (Pathfinder Society, but this won't be canon with the organization, obviously) that will be epic and amazing and whatever. I know that you all have a great understanding of the workings of the 3.PF game, so I'd like to ask you to help me come up with a challenging yet surmountable encounter for them. The group consists of,

Level 11 Gunslinger, Musket Master
Level 10 Cleric, unsure of the archetype
Level 10 Slayer, pretty sure it's just basic
Level 8 Bard, basic (this one would be my DMPC, so it doesn't matter if he dies)

The only other stipulation that I need is that most of the encounter comes from the original Bestiary for easy access. Other than that, any help desiging an encounter would be much appreciated, I can work with anything. I know that the challenge rating is somewhat misleading, so this would be a great help. Thanks!

[ExLibrisMortis]

I'm not very familiar with the PF bestiary, but I'm betting there's some appropriate monster/NPC that can kidnap and sacrifice the bard. The sacrifice opens a gateway to a world seeded by Ragnora, which has subsequently gone and used up all nutrients in the biosphere (Ragnora is an elder evil associated with corrupted, unbounded, destructive growth at all costs), meaning the entire surface is covered in a single dead organism--the sloughed-off world-skin--with parts of it either petrified, spontaneously animated as undead, or carrying dormant spores (which may drift through the gateway, emperiling the entire world, naturally).

That's just a little background for a quick jump to a post-apocalyptic world with potential for a lot of crazy dangerous environments. Most appropriate: undead encounters, plant encounters, undead plant encounters. I think PF doesn't screw over precision damage users like 3.5 does, so the gunslinger and slayer should be okay.