The Hadean Guard is the generic name for the 5th boss encounter in the Hell Eternal dungeon. Rather than one specific boss monster, it is actually made up of three unique monsters: The Iscariot, Brutus, and Cassius.
Each of these three monsters have an ability they can use if they are the last one of the group remaining. So, the key is to kill the ones with the most dangerous abilities first so that the last one remaining is the weakest of the three.
Iscariot's Immolation Invocation ability can be extremely annoying and completely removes a DPS from the fight while it is active, so he is typically killed first. Cassius' Metallic Purity is a reflect shield and so can make the fight drag on much longer as DPS back off while it is actve. Cassius is also able to heal the others or himself for 7K per use, so he is typically killed second. Brutus' Body Hammer only affects the tank and any melee DPS and so he is typically left until last.
So, the order should be 1) Iscariot, 2) Cassius, 3) Brutus
During the fight: