I have been playing PnP for some years now and also had one session GMing (my first) and I discovered that my biggest problem wasn't creating a world, inventing scenes or reacting fast if my players weren't following the intended path - It was to imagine what skills, spells and attributes my (enemy) NPCs had.

With every battle, I had to look up the attributes and skills of every enemy my players faced - and I couldn't decide which ones are appropriate to choose.

I tried to do this in advance, but I couldn't really imagine what would be hard for the group or what would be just boring because the enemies were too easy.

What is a good way of deciding how strong your NPCs are? I thought about just doing it on the fly, without any actual numeric representation of the NPCs' attributes, just decide that now would be a good time that this one dies and the other one gets enraged and does some more damage...

But that would totally kill the traceability of my actions - say, if a player wants to know if it was at all possible that this NPC had this-and-that ability --- because I don't really like to answer "It's like this because I say it is"... seems to bossy...

How do you deal with this?