Before reaching the Dimentionalist the party should kill off all five summoning portals, if these are not cleared when the encounter starts they will pour out mobs and make the encounter significantly harder.
This battle calls for two tanks, one to handle the dimentionalist and one to handle the waves of Guardian Fiends that will exit through the portal. The Guardian fiends die quickly, but can become a nuiciance to the healers and result in a wipe when not handled properly, and should therefore be killed whenever possible. The Dimentionalist doesnt cause much threat in terms of damage himself, so the key to this battle is managing the adds that come out of the portal. The boss himself is fairly tank and spank. He causes mild spell damage that can easily be countered by a tank and a healer.
The Dimentionalist uses a knockback move at the start, and in the middle of the battle, this can be easily told by the blue circle around his feet while casting. There is little that can counter this move other than moving outside its range, and once knocked back the Guardian Fiends can kill players if not dealt with before, The Dimentionalist will use this knockback move right before he turns invulnerable
His invulnerability can be easily spotted. He will surround himself in a massive bubble after a knockback. This will happen somewhere around 50% and only once. During this plase he will send out Guardian fiends in two's in addition to the normal spawns, these must be killed before the next wave of two Fiends come, and after that second wave the invulnerability will drop and the encounter reverts to normal.
During the last 10% the dimentionalist will turn to his gate and summon massive amounts of minions. If you are low on DPS these minions can overwhelm you, however a single AOE character can handle these easily. The Dimentionalist should die easily and you can enjoy the death of the first Amphitheatre boss and your new blue items.