While the Priest of Mithra is only available to the Aquilonians, it is an extremely important class to play. Why? Well, because this is simply the strongest healing class in the game, and they still have the abilities to deal some massive damage to mobs and enemies (just like a paladin in other games) - which makes them a great solo class. And, since they are able to carry any type of weapon in the game, and aren't limited to just a few (like in other classes), you can easily play this class even if you aren't used to playing a healer in game.
Since the Priest of Mithra can use any type of weapon in the game, it's only natural that they are proficient in all of the weapon types as well. So, when you play this class, you'll be able to train on one-handed edged or blunt weapons, two-handed edged or blunt weapons, crossbows, bows, daggers, and thrown weapons. This is great since you can then pick up anything and use it.
Now, keep in mind that this class is not a tank class, even though they can use any type of weapon in the game. The main goal of this class is healing their allies and using their holy spells to attack mobs. For this reason, you pretty much have a limited range of armor that you can wear. Cloth and light armor are the only types you can wear, but you do have the ability to use sheilds as well. So, this will help your defense rating. Even though you can only wear cloth and light armor, the cloth armor is actually the better way to go as you'll find these pieces will normally have buffs for healing or mana use on them.