I'm only just starting to study all the battle algorithms. The fact that your skill features the Beast element and your boss is weak against that element means whatever damage you do is going to be increased on him.
However, it looks like the algorithm also takes into account the ATK value of the weapon your character has equipped as well, even though you're using a skill and not a weapon. You might check what the weapon was that was equipped at the time, and see if the ATK on it is a little high. Also, if your character or the boss has any kind of statuses applied, that could enhance the damage you do.
Character agility and intelligence are also factors, and you haven't included images of those tabs.
I'd suggest to check the weapon, and if you don't think that would cause it, take a similar screen shot of the weapon stats, and those AGI and INT tabs for the character, and post here. Are you also sure your character is at level 1 when fighting this boss?