Omnipower is an ultimate ability associated with the Magic skill. It requires level 3 Magic. When activated, the user conjures a powerful spell that is enhanced with all four elements (air, earth, water, and fire) that, if it hits the target, deals 200% to 400% of the player's active spell's damage.
Although Omnipower's description and animation make it clear that all four elements are used, it will not be any more accurate against a target that is weak to a specific element. The same applies to Sever as animations themselves usually do not affect anything.