After each pact (mission) you are given a rank depending on how many points you are able to gain. You receive points for doing various things during the mission. "Legendary Sorcerer" is the highest rank and can be difficult to obtain, especially when you are starting out in the game. But there is an easy way to accomplish this:
Select the mission: Avalon Pacts-> I. Innocence--> Abbey Exterminators
Equip all Frost gear (sword, tri homing, etc)
The mission is to kill 10 Goblins (rats) and they are weak to frost. Just run in and attack all the rats and do your best to not get hit. You can finish the pact in about 90 seconds and if you aren't hit then the combination of points (speed, no-hit, vulnerability, minds eye) will be enough for Legendary Sorcerer. Rinse and repeat 50 times for the trophy.
Also, if you play in Multiplayer/Adhoc mode you can grind for other trophies at the same time such as 1000 sacrifices & offerings and games played in multiplayer.