Tertius 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 Tertius he summons adds.
At 75% he will summon 3 Rorarius.
At 50% he will summon 1 Gladius.
At 25% he will summon 1 Capsarius.
the Rorarius are quick and easy to take care of, the Gladius should be focused down and use freedom should he stun you, if you are stunned you will likely take damage from one of the lightning orbs. Finally when he summons the Capsarius kill it as quickly as you can, as it will heal the boss and reduce the damage he takes.
The best way to beat Tertius 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.