Masteria Expansions, Names and the proof for each

All bows are two handed.

Can't equip any bow and a shield at the same time.

Also notice that guns are held in the shield hand, while the normal "main" hand remains empty. In the case of this "monster", she has a glowing "main" hand, and a crossbow looking device in her "offhand".

But you're right. It could be a "shield" as well. But a shield that looks like that wouldn't necessarily be available as a player item, and it certainly would not be usable by Archers (all bows and crossbows take up 2 hands, and if anything a crossbow is more likely to require both hands than a standard bow). If it IS a shield, and it IS available as a player item, I'd say it would be to emulate the dual wield style daggers can get (via the jMS offhand dagger shields) but for Magicians. Can't say that sounds all that appealing to me... I'd rather a magician "dual wield" look to simply be a collection of energy in the other hand.

I didn't make mention of it possibly being a bow, I'm just suggesting it would be really cool if they managed to create a way to make a one handed crossbow.
Anyways, its not like it hasn't been done before.

There are other questions that need to be asked over the possibility of her using a gun.
Isn't the pirate or more specifically the creation of the gun in the maple "universe" something new.
Any information(from other versions quests) over when the pirates were formed as their own class might clarify this a bit, but until then its pure speculation.