This Seeking Tech guide will explain how you can get the Dragon of Legend trophy in Jeanne d’Arc. To unlock this trophy, you need to defeat Tiamat in Voldor Temple.
While you can visit the Free Combat version of the temple after the 26th stage of the campaign, Tiamat will be level 70. You most likely won’t stand a chance against it until your party is in the 60s. Instead, you should wait until you are in the postgame to head into the temple.
To prepare for the fight, make sure you have attacks and spells that can hit multiple areas. The Cyclone II magic spell from mages and Sweep attack from lancers are great against dragons and other big creatures.
During the fight, surround Tiamat on all four sides so that he can only attack one party member at a time. This should make healing a lot easier.
If your characters are dying in one hit or your party aren’t dealing any damage, then you are most likely still under-leveled. In that case, you should level up and then come back.