Who doesn't know it? You just go about the fight, do your best, 2nd in aggro and suddenly the tank is dead making you the bosses next target. For this occasion I present this armored glamour for casters. It won't raise your defense to tank level but you will at least look the part of a tank for these few attacks you may or may not survive. Enjoy your short tank life! :D
Weapon is optional, I just went with one that looks like it could hurt in close combat for the image