I failed this quest only once, because Jenlyns and Soulzglyl got swarmed by mobs and perished, meaning Duty Failed. You just have to stick close to them and fight whatever they're fighting. Also if they tell you to go after a specific target, follow their directions and change targets to whoever it is that they're talking about. Just make sure when the ACN appears at the end and summons his ugly pals, to go after him and take him down. It basically just comes down to sticking close to the NPCs and assist them.

And yes, the PLD quest is solo only - the walkthrough you were reading was probably outdated from version 1.0 which indeed could do it with a party. But in ARR all Job quests are solo-instances only I'm afraid.