The best way to do that is to break up the PT into groups with different objectives. Using current events as an analogy, the event would be like completing 5 different assaults at once. After completion, the group reunites to fight against a NM.
To expand on this. The Objectives would be split up into different categories. Imagine yourself entering a room, and You're in 3 parties. Its an Alliance fight. You would have the option to go straight for the boss, Or to target side objectives that would do things like
1) Improve boss drop rates
2) Weaken the bosses Defense
3) Weaken its offense
etc, as well as provide a small chance to drop the item the bosses drop - ala Einherjar/Odin.
The Categories would be designed around different party situations - Mage, Tank, DD. For example, If you have a mage party take the mage path, It will be a trial of your knowledge on Enemies.
You'd spawn in an area, in front of you would be some enemies, Different families, etc. They would aggro after you nuke them. If you nuke them with their weak element (I.E if you hit a crab with Ice) they will die instantly, If you nuke them wrong, They will proceed to aggro you. These enemies would be strong defensively and offensively, But not impossible. Knowing their weakness will save you trouble. Hitting the Enemies Weakness will also activate the bonus.
Another possibility is a room with multiple enemies, testing a mages ability to crowd control and kill. These are just examples of a Mages room.
You enter in a Room, There's three paths. Labeled "Mage" "Melee" "Tank". These would be objective based rooms only 1 party each could enter. Balancing your parties would still be the best idea, But the Objective would vary depending on the room, given you knowledge on what to expect going in. You can take a DD party into mage room, But it will be harder. You can enter all 3 Simultaneously.
Mage side objectives would be.
1) Bypassing enemies through Sneak/Inviisble - ala Maze Monger.
*Perfect clear would be getting through undetected.
2) Randomly spawned Enemies, Nuking with their weak element kills instantly,
Wrong element causes aggro.
*Perfect Clear would be 1-shotting all enemies.
3) Multiple Enemies, testing your ability to Crowd Control/Kill.
*Perfect Clear would be taking Less than 1k Damage total.
4) Every so often Enemies would Be hit with a phantom Skillchain - Any Magic Burst kills instantly.
*Perfect Clear would be Killing more than 5 enemies with a Magic Burst.
Melee Objectives.
1) a "Blitzkreig" style wave of Enemies. Easily killed. Tests your survival and damage.
*Perfect Clear would be killing all within 5 minutes.
2) A Mini boss, Weak in all, High Defense, Easily killed by overpowering its defense with your attack.
*Perfect Clear would be winning within 5 minutes.
3) Enemies spawn randomly, Hitting them with the Physical damage they're weak too One shots (Slashing, Blunt-Staff/club, Blunt-H2h, Piercing), Hitting wrong aggroes them.
*Perfect clear would be one shotting all Enemies.
Tank options. (All have high PDT/MDT, But can still be killed through melee) (Ninja Works well in some rooms, PLD more in others)
1) Strong enemy dies slowly to DoT, Tests your Survivability.
*Perfect clear would be never dropping below 75% HP.
2) Enemy attacks slow, Takes multiple shadows, Hits hard, Easily remedied by Defense, or well timed Flash, or keeping shadows up. Dies slowly from DoT.
*Perfect clear would be never dropping below 75% HP.
3) Shield Blocks will damage enemies significantly, Enemies PDT/MDT Removed.
*Perfect Clear would be Doing the majority of Damage from Shield Blocks (For PLD). Or Receiving no damage at all (For NIN)
4) Multiple Enemies in waves ala Blitzkreig, Die slowly from DoT as well as damage reflect from shield blocks, Tests your crowd control/Positioning skills (Keeping enemies infront of you for shield blocks) Enemies have Low accuracy as well.
*Perfect Clear would be keeping all enemies in front of you for the majority of the fight.
**(Dies slowly from DoT = about 2 minutes per fight)
These are spitballed ideas, Not really incredibly thought out - But the idea is each room would have a specialty where certain jobs would shine, and your knowledge and abilities will be tested. Each room rewards you with something when you successfully complete it. "Perfect" Clears would add bonuses.
Completing a mage room would weaken the enemies Magic Defenses. "Perfect" Clear lowers it magic attack.
Completing a Melee room lowers its Physical Defense. "Perfect" Clear lowers its Physical Attack.
Completing a Tank room Increases its drop rate. "Perfect" Clear adds an additional drop slot.
You can however, Ignore all options, and shoot straight for the boss if you're in a hurry.
--- Moving on to the boss. Boss is say, a Manticore. It has...
200,000 HP. (??)
Special Abilties:
Physical Attacks ignore PDT. Defense effects enemy heavily. Takes enchanced damage from "Reprisal" (2x-3x)
Fight begins with Enemy in the center of the room, no special abilities at start. Has a decent attack speed, and good double attack. Can cast high level magic. Every 1-2 Minutes, It will move to a certain side of the room. As shown above.
*North Side gives it a Strong MDT (90%)
*South Side gives it a strong PDT (90%)
*East Side gives it a Chainspell Effect - Immunity to Stun.
*West side Gives it a Hundred Fist effect -Immunity to Stun.
*Center of the room - Benefits from all Auras. Gains access to "Weapon Break".
North is triggered by heavy Magic damage over the course of the 1-2 minutes, South is from Heavy Physical damage, East is from Stunning too many spells, West is from Stunning too many TP moves. Remains stationary within the "Aura" for duration, Has Draw in. If Draw in is used, It is followed by a Tail Swing. - The directional dynamic of this fight encourages a diverse party set up, of Melee, Mages, and a strong tank (To avoid stunning too much).
Moving to the Center of the Room is triggered by dealing more than 20% of its HP within 30 seconds. Which would only be possible through a zerg. This would be an Anti-Zerg tactic.
Has the following TP moves.
Heatbreath - Single Target very powerful Fire damage. Used when another Player takes hate.
Scorchbreath - Conal fire damage, Strongly reduced with high defense.
Tail Swing - Single Target. Used after draw in.
SandTyphoon - Conal Earth Damage, strongly reduced with high defense.
Roar of Eternity - Gains Haste, Attack, and Magic attack bonus, Enemies in range inflicted with Terror for 5~10 seconds.
Sphinx - Reduces HP and MP of enemies in range by 80%, Ignores current Target.
Weapon Break - Reduces Attack and Magic Attack of targets in range by 85%. Used only if extreme damage is down in a short time.
All moves can be Shield Blocked as well. If blocked, Reduces damage and/or Negates additional Effect.
Every 75% HP, He will use Roar of Eternity, Which boosts his attack/magic attack/haste by 50% each. It lasts up to 3 minutes. However, Each successful shield block will create a "Reprisal" like effect, lowering the enemies attack, Magic attack, and Haste by about 2%, Reducing the impact of this significantly if you have a good Paladin.
As for how Defense will play a role, It will have a high base damage, but only moderate attack, and its pDif will be able to fall below 1.0, So stacking Defense, or as a PLD, your natural high defense, Will grant you much stronger resistance to the enemy, where as a normal DD, due to it bypassing PDT on physical attacks, will get hurt much more significantly.
A Normal DD could stack DEF, but a PLD would be able to do it more easily. Its not a fix to PLD, just an example of a fight that could encourage a more dynamic "Team Play" element with balanced parties.