I just beat Diablos with a pick up pt and here is how it went down. ( i am not gonna go through every detail cause if you are on this mission then you prob. already know what to do and if you are not then you will have no idea what i am talking about anyway.)

1. After farming a chip for someone who was clueless on how to start the mission we headed to Pso'xja. (40 cap)
2. We ran from zone to zone. ( red - black - red- black)
( You more or less run trough a red/black cloud )
( this part takes about 7 mins )
3. We zoned into shrounded Maw.
4. Went over strat (brief version)
- Buff at start
- Know what tiles to stand on
- stun nightmare
- kill
5. -Full Attack - Diablos
6. PT setup was : WHM - heal
BLM - dmg
BLM - dmg
NIN - tank
DRK - stun
SAM - stun/dmg (me)
So we went in and attacked. He tried to do his first nighmare at 85% hp. I used WS (tachi:haburk) to stun and stopped him. Then used 2hr and meditated to create 2 distortions in which both of our blm's could MB on to create lots of dmg. Kept killing him till he was about 65% hp. He did nightmare again and Drk casted stun to stop it. At 60% hp he moved to floor and lost our drk. So we are now down to 5 ppl left. Nin kept tanking and doing a good job at it too. We kept beating away and BLM's kept nuking it. He then moved the floor again at 30% hp where we lost our whm. At 20% he did Nightmare again and I used WS to stun. We just kept kill it and BLM's kept nuking till dead.

2hrs used: SAM
BLM
BLM

There is nothing special needed to do this other than basic mage and melee food.
Melee food used: sushi ( cause missing sucks )
Mage food: Rollenberry Pie, Yag drinks(x2-3).

At one point I remember 1 of our blm's resting for a short time for MP but other than that there was no resting or pause in battle for anyone.

No need to over-analyze. Just bring someone with stun.
We had 2 people. (2/3 nightmares I stunned with WS but i would still bring drk just to be safe even though a dedicated sam could stun nightmare)

Sam stun abilities:
WS = tachi:Hobaku
-Mediate while buffing
Stun with WS the first nightmare
-Meditate and just save TP to stun nightmare again
-Attack to gain TP for stunning again
- You can always use 2hr and create 1 distortion and save the last 100tp for stun
Note: Stun actually works and renders Diablos useless for about 10-12 sec.