At level 15 you gain the ability to completed in 'instanced raids' These are your dungeons you unlock as you gain levels and unlock them. Both directly selecting the raid, as well as your dailies such as low level, high level, and expert. Do this 100 times to unlock the trophy.
Side note: As someone coming from the PC version as well I had completed over 100 instanced raids. In order for this to unlock you will need to complete just 1 dungeon and it should unlock.