The Mage Arena minigame is where you get your god capes from. You need these capes in order to use the Upgrader at home to get your God Capes 2. The minigame itself is fairly simple but if you go in without knowing what order to kill the NPC's you might be caught off guard with the damage. You can get to the minigame by type ::mb and talking to Kolodion. You will need a total of 420 Mage Arena points in order to get one of the god capes. You can also get infinity pieces and god staffs here.
The Fight
The easiest way I've found to average kill times of under 2 minutes is to simply go into the fight with your range weapon of choice and to pray Magic. Focus the level 79 Elite Mage first as he does by far the most damage and freezes you in place. When the level 79 mage is done, feel free to switch into your melee gear, turn on 
and just kill the other 2. The fight is incredibly easy once the mage is dead.
Alternative Methods
Since the NPC's all pray magic, You could realistically go in with just range and do it that way as well. As long as you kill the Mage first you should be fine. I'd advise bringing 8-10 food of your choice just in-case. Magic shortbow with rune arrows or rune crossbow with Diamond Bolts (e) will be more than adequate.