Board Thread:Game Discussion/@comment-24.165.223.70-20130929235411/@comment-23870891-20130930064624

In arena the randomness of your opponents makes Dual both better and worse then mono, more chances for bonus damage, but more chances to take bonus damage in turn.

It matters more against the weekly bosses, and then it depends on the boss:

Agains a mono element boss dual element is slightly better solely because of the guild bonus perk.

Against a dual element boss with the elements beside each other a dual element can hit both boss weaknesses without being weak in return (i.e: Spirit/Earth vs Fire/Spirit causes boss to take bonus from both fire beating spirit and spirit beating earth), and you also get the guild bonus perk to both elements, making it significantly better.

Against a dual element boss with the elements separated (i.e: Fire/Wind), mono may actually be better due to being easier to control. To use dual element (For guild bonus only) against such a boss you need the exact armor to avoid being weak to boss while still dealing some bonus damage (Earth/Wind to counter the above example). However, it is impossible to get double bonus without exposing yourself to more damage in return (Try to use Earth/Water against the Fire/Wind boss and you will take extra damage from water being weak to wind). Given all this, its simply easier to use a mono-element.