Here's my advice...
The most min/maxed characters are all fighter types,
Which makes them great to dominate/charm/stun/whatever with Will-based magic.
Those fighter-types are even better when turned against the party they're supposed to be fighting for.
Be sneaky...have one monster as a distraction, while a hidden one dominates from a distance. A fighter type is going to fail his save eventually...
He also uses the Whirlwind/Great Cleave combo to kill any bad guy surrounded by low level guards.
Well, then...use missle attacks.
Put a pit between the foe and the war machine...forces them to use their paltry little ranged attacks.
The Arcane Trickster is an NPC.
Ah, this makes it easy then...kill her off. Preferably in a horrid and malicious sort of way. A way that makes the PC's realize they're *not* invincible.
Without Sneak Attack, the rogue-types do nothing compared to the fighters. With sneak attack, the rogues destroy anything that they can hit.
...except constructs and/or undead. A simple Animate Objects could keep them and their little sneaky attacks busy long enough to take their piddly HP out.
The OOBI is a half-celestial, which gives him spells and makes him very hard to kill. At close range he does as much damage as a melee fighter
All the more reason to use ranged attacks against these guys. Use weird angles or stealthy foes...shooting a bow doesn't make you visible, and if they're high enough in the Rogue department, they could be dishing out sneak attack damage too. As a half-cel, the guy's gotta be pretty wussy on HP. If he goes down, and the trixey one's been killed horribly, then all you have to deal with is fighters who can be dominated.
The Frenzied Berserker has such ridiculously high Str that Supreme Power Attack is a worthless ability, since he already does over 100 damage a round to anyone with a low (30) AC.
...as well he should probably do. Which is why you need to use sneaky ranged attacks and cheap lil' spells. He's a berserker...he'd be a fun guy do Dominate.
In general, I don't see a *whole* lot of problems, here. Fighters are supposed to be dishing out a lot of damage, ditto with barbarians. Ninjas and Rogues are supposed to be basically sneak-attack-and-then-run-like-wussies.
Some general guidelines for tactics...
1) Put something between the party and the attacker. Hiding foes firing bows, arrow slits, a long trapped corridoor, a pool of magma, whatever. Large numbers of creatures spaced far apart who hit them at range...nickel-and-dime 'em to death. Without a healer, they shouldn't have too long.
2) Be sneaky. Hiding, darkness spells, illusions...make them waste their few precious rounds of maximum buffness. Spells don't last forever, and there's only so many POW spells they can cast.
3) Hit 'em where it hurts. Their saves. *every* class has at least one weak save that a savvy opponent will know and take advantage of. Fighter-types are usually vulnerable to Ref and/or Will. Rogue types are vulnerable to Fort and/or Will. Mage types are vulnerable to Ref and/or Fort. Hit Fort and Will most often, since those tend to have the worst consequences (getting stunned, poisoned, drained of levels, whatnot).
a) Fort weakness: Stunning, poison, outright death, draining levels...if you ever want to blunt those sneaky attacks, Undead would be a lot of fun on these guys.
b) Ref weakness: Basically area-damage territory. Certainly doesn't hurt to take them down a few notches while they're still closing in.
c) Will weakness: It's time to make the enemies fight for you. Most will-based saves take combatants out of the arena for a few rounds at least, and, at the higher echelon, they start getting into coup de grace territory...or etenral slavery. Either or.
4) High HP? Not after HARM/METEOR SWARM, it ain't!
5) High AC? Hooray for incorporeal!
So...I guess what I'm saying is send 'em after a cabal of vampires (undead and sneaky with a charming gaze and level drianing powers) who perhaps have Wizard/Sorc levels (for more versatility), who like to use automatons (including traps, if you'd like) and illusions to weaken up prey, and will never, ever enter into a straight-up combat. Ideally, they'll splinter the group into several smaller parties (or individually) with traps, spinning walls, illusiary doors and the like that they can then take their leisure killing off the PC's one-by-one.
That's right. Gnomish vamires. Cowardly gnomish vampires of doom, with little robotic familiars!
After wiping out a PC or two, you should feel better, and so will they.
Then, when they buff their newfound weaknesses, they can get smacked in other ways.
IMHO, using the core rules, there's *always* a way to take down a PC, if you have the motivation. A combination of rat-bastard DMing and brilliant enemy tactics will hurt them much.
Oh, and if you go by CR, expect them to be able to whoop about 5 encounters of their CR before feelin expended. CR isn't meant to be a great challenge, just something that'll...y'know...make them use a potion or a spell or two. You probably want to go at least two above for one creature of "boss-level" power.
