Quintus will attack with magic, so using Deflect/Protect Magic is recommended.
All the Legiones share the same Special attack, they shoot a ball of lightning into the air and it will land on the ground in a 3x3 square area lasting for 3 seconds. If you stand in its area of effect you will take constant damage for around 2000.
Each of the Legiones have a special effect each 25% of their health they lose (75%, 50% and 25%) for Legio Quintus he will 'draw' a line across the room the line will spawn under your feet and stretch from one side of the room to the other. These lines effectively cut down the room you have to move around in. These lines deal large damage when you cross them, never run along a line as it will deal continuous damage.
The best way to beat quintus is to keep moving, once you have him targeted you can run around his room in a circle pressing your abilities. Try to run along the walls so that the Lines will not cut off to much of the room, and only cross them if you have no other choice.
Try to avoid using combo abilities as they will prevent you from moving and you will be hit by one of the lightning balls and take a lot of damage.