While this clicky lands on many resistant targets, the best it can offer is 1413 dmg in 99 seconds (15 ticks + cast time, *IF* the target lives 99 seconds), I think a better option for mana-free DPS would be the clicky on Sacred Shield of Dread: Drain Soul. This has several advantages: MUCH better range to begin casting (can do this on incoming, at max range, with very little chance of pulling aggro by the time it lands), it does all of its damage at once, meaning you get more damage from just one click if the target isn't certain to survive the 99 seconds needed for Curse of Spirits to run its course. Furthermore, you're healed for the amount of the nuke, if you're below max health, and it scales with your level! At level 60, it lands for 406.
Non Iksars stick with Jaundiced Bone Bracer, cliky nuke Winter's Roar with a cast time of 8 seconds. This gets resisted most often of the three, and for me (at level 60) always lands for 246 damage.
Bracer = 246/8=30.75 DPS sustained if you chain cast, assuming your target isn't especially resistant to cold.
Shield = 406/13.5=30.074 DPS sustained if you chain cast it.
Epic = 1413/99=Only 14.273 DPS, and ONLY IF the target survives the full time
More tellingly, after the 13.5 seconds have past, even rounding up to the next tick, the epic has only done 34 dmg vs the full lump DD of the shield. Given the ease with which most could complete Sacred Shield of Dread, I think it's worth considering if you're on TLP. After 13.5 seconds, the bracer has dealt probably 246 damage, and is half way to doubling with the second cast.