RDM tanking in my experiences is a lot of fun! To do so successfully you will need a lot of patience and practice. The first thing you will want to do is get as much VIT, DEF, and -physical damage gear as you can. Second is get the highest DEF "type 1" shield you can, this will make a HUGE difference; it doesn't matter that the "type 1" shield blocks less damage per hit because it blocks more often and with Stoneskin and Phalanx on, the difference becomes null except in the case of a critical hit. Keep your opponent/s in front of you to allow the shield and parry to proc. Shield and parry might not be much in and of themselves but every "grain" of sand will be added to cement a wall of defense. Keep Aquaveil on at all times; along with -spell casting interruption rate gear(macro swap in your Warlock's Tabard for Stoneskin) and merits. You really only need for Stoneskin to last until it can be recast; if it lasts longer than that, it's just icing on the cake. Campain battles are a great place to practice tanking especially so if you can get your hands on some of the Campain specific gear. Icespikes are a RDM's best friend along with a pair of Duelist's tights. Use Enaero II to lower you foe's resistance(if any) to ice magic so that your ice spikes will have a probability of greater effect. To engage your enemies with "Composure" I think says a lot about what it takes to tank as a RDM. The ability is amazing by itself, but the real magic is in the meaning.