Lesson Two: Numbers and Rolls
As I said before, battle systems, which GMs normally deal with, vary greatly. Across the board, though, they tend to use dice/RNG rolls to determine if actions succeed or fail. I personally use HibkiKuze's system of using a d20 (20-sided dice) for hits and misses, with 5 or below being a miss- of course, this can be altered by buffs/debuffs.
I suggest not creating stats for enemies to start off with, and instead judge based on attack tier (-dyne, etc.) and health. Percentages is a pain for me, but good if you don't want to implement a HP system. I suggest dividing it into fractions: for example, 6/6 means an enemy can take 6 base level spells, 3 mid-level spells, etc. But that's just me. As for players... let them have the "honor system" at first. Even experienced GMs trust players to be honest with what their characters can dish out or take. However, don't be afraid to put your foot down if someone spams Armageddon or something.
Also, don't worry if all this system stuff is confusing! It takes a while to get used to, and you'll eventually develop your own style. Besides, not all GMs have to use a battle system- especially if they're more narrative or exploration-based.