Secundus 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 Secundus his attacks get faster.
The best way to beat Secundus is to keep moving, once you have him targeted you can run around his room in a circle pressing your abilities. 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.