Guardian

Guardian can be summoned with the Level 5 Sorcery power Summon Guardian.

Overview

Guardian can be summoned with the Level 5 Sorcery power Summon Guardian.
A Grand Guardian, along with a Grand Watcher and a Grand Fury, may be summoned with the Grand Summoning power.

The Guardian used to resemble a burning skull with horns, but this was later changed into a golem-like creature. This also removed any visual differences between the Guardian and the Grand Guardian.
It is highly advisable to use Guardian with Offering/ M.E.R.C.Y. Attack Protocol and Arcane Battery [Passive Ability] if your group doesn't have a Tank. Additionally, when pairing the ranked-up M.E.R.C.Y. Amazotron Matrix with Stack Overflow [Passive Ability], Passive Ability 2 - Combat Suture, The Clarion and Mystic Symbol Of The Seven, the Guardian can receive healing and weak shielding easily. For the constant normal shielding of M.E.R.C.Y. Defensive Module, consider summoning an Accomplice, Henchmen, or Trinket Pet every 60 seconds.
The only difference when using M.E.R.C.Y. Attack Protocol over Offering is it reduces the cooldown of your Trinket Pet.