I need a lot of easily-playable builds

[Kazyan]

So...I have a dungeon, of sorts. The PCs have previously found, through artifact-powered Mindsight, that there are various guardians around each of the five nouns they're supposed to verb to get to the final boss. All of this is at the ocean floor. They have the relevant spells and powers to get there.

I need at least 10 Merfolk of about CR 20. They can't all be the same build, or the PCs will get bored/snark at me. Templates allowed, provided their type does not change from humanoid, or a continuity error results. Paragon is not allowed because I promised the PCs I wouldn't do that again. (A shark was involved.)

The builds cannot be particularly complex to play, because I'm already running an NPC which the PCs are dragging along with them, and I will be playing multiple of these merfolk at once.

Bonus points if you can prevent the merfolk from dying immediately, despite being under-CR'd for this group. These are optimized PCs that were able to handle CR 23 creatures at level 17; they are now level 18 and just picked up a smattering of 9th level spells. The PCs are a summoning Archivist, a golemficer (I'm not sure if the golem will be taken into the dungeon), a thrallherd Erudite, an ubercharger, the thrallherd's DMM: Persist cohort, the ubercharger's force-specialized Sorcerer cohort, and their pet NPC Factotum/Chameleon with plot abilities.

[DeAnno]

Are they fighting them one by one or all at once or in small groups?

Metamagic blaster Psion and metamagic blaster Wilder set up to blow their loads ASAP are probably both good fits for this. In general, if you want to make things hard at this level without being impossible the simplest way to do it is threaten their buff stacks a lot by having enemies that throw Disjunctions (and/or Chained Metamagic Greater Dispel) around.

[Kazyan]

In small groups; they will be divided into five teams. I might bunch the last two groups together if the PCs steamroll them too easily.

[Biffoniacus_Furiou]

Temperate zone, tropical, or cold/arctic? What's the party's/opponents' alignments? Going to need a bit more background on the opponents. Would the Voidmind template from MM3 be fitting? What about Dark Creature in Tome of Magic, or Shadow Creature in Lords of Madness? What about Toschar from Lords of Madness chapter 7 controlling merfolk hosts?

Shadow Creature Merfolk Assassins:
Shadow Creature Merfolk, Spellthief 1/ Psion 4/ Psychic Assassin (http://www.wizards.com/default.asp?x=dnd/psm/20040723d) 5/ Slayer (http://www.d20srd.org/srd/psionic/prestigeClasses/slayer.htm) 9 (CR 20, ECL 22)
Size/Type: Medium Humanoid (Aquatic)
Hit Dice: 4d4+6d6+9d8+95 (169 hp)
Initiative: +6
Speed: 7.5 ft., swim 75 ft.
Armor Class: 46 (+6 Dex, +11 armor, +6 shield, +4 natural, +4 insight, +5 deflection), touch 25, incorporeal touch 42, flat-footed 40
Base Attack/Grapple: +14/+17
Attack: +1 Sword of Subtlety, Speed, Wounding, Marrowcrushing: +25 melee (+29 sneak attacking), 1d6+4 plus 2 Con damage (1d6+8 plus 2 Con damage plus 3d6 plus 2 Int damage sneak attacking), 19-20/x2 or melee touch +24 or ranged touch +24
Full Attack: +1 Sword of Subtlety, Speed, Wounding, Marrowcrushing main hand: +23/+23/+18/+13 melee (+27/+27/+22/+17 sneak attacking), 1d6+4 plus 2 Con damage (1d6+8 plus 2 Con damage plus 3d6 plus 2 Int damage sneak attacking), 19-20/x2 and +1 Sword of Subtlety, Speed, Wounding, Marrowcrushing off-hand: +23/+23/+18 melee (+27/+27/+22 sneak attacking), 1d6+2 plus 2 Con damage (1d6+6 plus 2 Con damage plus 3d6 plus 2 Int damage sneak attacking), 19-20/x2
Space/Reach: 5 ft./5 ft.
Special Attacks: Sneak attack +3d6, mind cripple (2 Int damage per sneak attack), death attack DC 23, poison use, steal spell, psionic powers, favored enemy (choose a psionic race) +6, breach power resistance,
Special Qualities: Amphibious, low-light vision, +2 save vs poison, enemy sense, brain nausea, lucid buffer +4 vs compulsion and mind-affecting, cerebral blind, cerebral immunity, resist cold 15, darkvision 60 ft., shadow blend, evasion, fast healing 2, +2 luck bonus to saves, plane shift 1/day (to or from the plane of shadow only), DR 5/magic; (buffs: mind blank, true seeing, touchsight 80 ft., freedom of movement, resist acid, cold, electricity, fire, and sonic 30)
Saves: Fort +22, Ref +28, Will +23
Abilities: Str 16, Dex 22, Con 20, Int 26, Wis 8, Cha 8
Skills: Listen +21, Spot +21, Swim +33, Hide , Move Silently , Psicraft , Spellcraft +10
Feats: (two flaws, insomniac and love of nature, no noticeable effect) Darkstalker (LoM), Practiced Manifester: Psion (CP), Psicrystal Affinity (Coward), TWF, Track, Extend Power, Persistent Power, Mage Slayer, Weapon Finesse, Psionic Meditation
Environment: Temperate aquatic
Challenge Rating: 21
Treasure: Equipment
Alignment: Any Evil
Advancement: By character class
Level Adjustment: +3

Daily Buffs which are manifest before resting, bonuses included in the stats above; costs are if he needs to renew them:
Mind Blank, Personal (13 pp, 24 hours)
Persistent True Seeing, Psionic (17 pp, 24 hours)
Persistent Freedom of Movement, Psionic (15 pp, 24 hours)
Persistent Touchsight, augmented to 80 ft. (17 pp, 24 hours)
Persistent Mental Barrier, augmented to a +5 bonus (17 pp, 24 hours)
Persistent Energy Adaptation, Specified, five times, 30 for each energy type (55 pp, 24 hours)
Persistent Force Screen, augmented to +6 (17 pp, 24 hours)
Extended Inertial Armor, augmented to +11 (17 pp, 36 hours)
Persistent Defensive Precognition, augmented to +4 (18 pp, 24 hours)
Persistent Offensive Precognition, augmented to +4 (18 pp, 24 hours)
Persistent Thicken Skin, augmented to +4 (18 pp, 24 hours)
Extended Share Pain targeting the psicrystal, don't forget its hardness 8 (5 pp, 36 hours)

Basic Equipment:
Circlet of Rapid Casting (MIC, transparency), +6 Enhancement bonus to Int (MIC p234)
Gloves of the Balanced Hand (MIC), +6 Enhancement bonus to Dex (MIC p234)
Banner of the Storm's Eye (MIC, shoulders), +5 Resistance bonus to saves (MIC p234)
Choker of Life Protection (MIC), +6 Enhancement bonus to Con (MIC p234)
Charm of Earth Spell Heightened (10th) Deeper Darkness, NPC spellcasting (spell level 10th, requires a light spell with a spell level of 10th to cancel it, or 11th to overpower it)
Two Swords of Subtlety with Speed, Wounding, and Marrowcrushing (BoVD) added and a wand chamber (DS) in each (+9 weapon + 20,000 gp + masterwork shortsword + wand chamber)
Wand of Wraithstrike (SC), 30 charges, in a wand chamber
Wand or Dorje of something useful in the other wand chamber
Two Healing Belts (MIC), one on the Psicrystal. Psycrystal is in a compartment on his person so opponents don't have line of sight/effect to it and cannot target it directly or hit it with area effects.
Anklet of Translocation (MIC)
Mundane ink capsules to negate all visibility in current and adjacent squares when activated.

Psion (Egoist) 14 Powers, manifester level 18, 242 powerpoints/day, DC 18 + power level + augmentation, 27 powers known (5/4/4/4/4/3/3):
1st: Force Screen, Inertial Armor, Defensive Precognition, Offensive Precognition, Thicken Skin, Vigor
2nd: Energy Adaptation Specified, Share Pain, Energy Stun
3rd: Touchsight, Mental Barrier, Body Adjustment, Hustle, Time Hop, Dispel Psionics
4th: Freedom of Movement Psionic, Telekinetic Maneuver, Intellect Fortress (transparency if possible), Dimension Door Psionic
5th: True Seeing Psionic, Psychic Crush
6th: Restoration Psionic, Disintegrate Psionic, Temporal Acceleration
7th: Moment of Prescience Psionic, Mind Blank Personal, Oak Body

Tactics!
Prior to combat: Extended Moment of Prescience, Psionic (15 pp, +18 initiative), Oak Body (current natural armor bonus is an enhancement bonus which will stack), Vigor augmented to grant 90 temporary hp to both himself and his psicrystal.
Surprise round: Psionic Disintegrate an obvious spellcaster (spell component pouch, divine focus), don't forget sneak attack, or Psychic Crush an armored opponent with no spell components or divine focus, or Dispel Psionics if opponents are known to use a lot of buffs. Temporal Acceleration, get within melee, activate an ink capsule.
1st and subsequent rounds: Activate Wand of Wraithstrike (same as the spell's casting time per Rules Compendium), full attack with sneak attacks until the opponent stops moving, then switch targets. Temporal Acceleration to escape or rebuff if dispelled, to refresh Vigor, to use an ink capsule, etc. Temporal Acceleration rounds don't count against the duration of Wraithstrike, so he can use that at the start of a round, do what he needs to and activate Wraithstrike during the TA, then when he returns to normal time he still has Wraithstrike for that turn.

[Kazyan]

Temperate zone, tropical, or cold/arctic? What's the party's/opponents' alignments? Going to need a bit more background on the opponents. Would the Voidmind template from MM3 be fitting? What about Dark Creature in Tome of Magic, or Shadow Creature in Lords of Madness?

The setting is the ocean floor--exactly 4 degrees Celcius.

The PCs are: CG, CE who has CN written down on his character sheet, CE, and NE. Cohorts are NG and...CG, I think.

Opponents are LN with some wiggle room, because they are a set of elite guardians of an all-but-dead "deep merfolk" settlement. They've bound an advanced Titan in temporal stasis and are using an energy-flavored artifact he's carrying to power five barriers around him. Breaking them requires the PCs to destroy five pentagonally-arranged orbs of energy (which are a riverine casing over homebrew "energy banshee" monsters). The Titan is being sealed because he is Bad News for the settlement and the campaign setting in general; he terrorized the oceans with the artifact in the past and has been locked up since nearly the beginning of recorded history. He can survive underwater for reasons I haven't entirely hammered out as a DM. His artifact is a weapon crystal with the power to channel all five energy types.

Voidmind would definitely work, because the guardians have recently come under the influence a Pseudonatural (epic template) Ulitharid who has an interest in keeping the artifact there--he considers it his own item, but doesn't have a use for it; he will show up after enough distress calls get sent to him via Sending Stones.

Dark and Shadow would work, since it's the ocean floor.

Any concerns about pressure damage are handwaved with a cosmetic change to the merfolk and describing them as "deep".

[Biffoniacus_Furiou]

I edited my above post, but it took forever because I'm at work. That's liable to outright disable multiple PCs, if it doesn't wipe out the whole party on its own. Note that targeted spells and abilities cannot be used against an opponent with total concealment because you do not have line of sight, but effects that use an attack roll still work, you just have to target the right square and there's a 50% miss chance. Touchsight is amazing.

[Kazyan]

*reads through* Awesome, BiFu. That will be able to handle one of the five points on its own. I'll tell you how it goes.