It's difficult to take this seriously with the actual enemy they are fighting. It's a fucking furball. It's utterly shattered my suspension of disbelief.

I'm trying to figure out the questions being asked, but here is my attempt at answering.
As far as it goes, he had to use skill points in order to get his skills activated. So far, he has attack up and defense up, which took up his two skill points he already had.
Based on how it goes, it seems to have a time limit. So, while not limited to a single use, it may not last the entire fight either.
And as for why she wasn't in the party... it seems she was never part of any party in the first place when she went into the dungeon, so she may have met them while in there shortly before encountering the redface.

Edit: Yeah, you can see she wasn't part of their party from what the dude said in the bottom panel of page 6(note, including the cover and warning pages), they were escorting two rookies in the dungeon who seem to be the ones cowering on the ground. Manager-chan just went out of her way in order to help them survive the encounter.

The hell that round red mouse thingy was laughing at?? And you can just gain skills by thinking about them?
Why the hell aren't you buffing them??

Why does MC have to cast the defensive skill everytime there's an attack? Is the skill only one time use? It's not a continuous buff?

@Saviticus she probably didn't trust the dude seeing how vulger and rude he is

Seriously, thank you for this translation. It makes so much more sense, and does a much better job of explaining party dynamics and the relationship between MC and Manager-chan.

@Saviticus

I think it's there but please be patient for the release :D

Getting close.
Also there's this huge plot hole I'm looking at and its "why wasn't she in their party in the first place?" Hope the next chapters explain.