Okay, so you need to understand how cheap Luna is. As a rule, all bosses are stupidly hard to kill by nature. In Fire Emblem, that means that they have massive defense and resistance stats, which dictate how much physical and magical damage you can mitigate respectively. A defense of 12 means that 12 points are subtracted from the enemy's attack, and if the attack is less than twelve, you take no damage. (Attack is made up of unit strength plus weapon might.) Same with resistance. So bosses usually have something like 21-25 of each stat later in the game, always increasing from map to map.
The dragon was like that, about two points shy of max (30) in each stat, with an additional +10 across the board. (Strength, skill, speed, everything.) Since no stat in Fire Emblem can go higher than 30 (20 until you change classes), and every stat increases one point at a time (even HP), adding 10 points to everything is absolutely nuts.
Now, in the description of the dark tome Luna, there's a line that reads "negates enemy resistance". The tome has no might of it's own, so that basically means the user inflicts it's own magic strength as direct damage. Early on, when Shamans are low-level, the tome is useless, since enemy resistances are so low that the lowest-tier dark tome combined with the user's magic is enough to do much more damage. But late in the game, when I've shifted my Shaman to Druid and raised him to about level 19 (one shy of max), and he has 22 magic power, using just that stat is much better than trying to overpower the enemy resistance.
In other words, I'm taking the boss's boss-level stat and telling him he no longer gets to use it. Cheap as heck.
So back to the dragon, I see Hector with double 25s (not quite sure how that worked out; I mean, I know Hector's buff, and all, but I think the axe he was using doubles its might against dragons or something) and Canas with 22s and decide to send Canas in first. I also notice he has about a 9% chance to critical hit, and an 86% chance he'll even hit in the first place.
He criticals.
Twice.
Bye-bye dragon.
Critical hits mean the damage triples. So think 22 per hit, and that becomes 66 per hit. That's 132 points of damage, and the dragon had 124 and (and this is the best part) couldn't kill Canas in one blow. So Canas basically ripped a hole in its chest with the first strike and tele-grabbed its heart with the second.
