There have been several shark and
barneytojil runs I've been on, 6~18 man, where the only reason we did not wipe was because I was in the DD party on DNC, and when the WHM was asleep or out of position and stoned, or the Stunner was asleep or stun resistance was up, was me popping trance and spamming Waltzes with Fan Dance up.
Can't do main healer, as there's too many status effects Healing Waltz doesn't remove, and if you're stoned without a WHM or /WHM to stona you, that's it.
I have, however, soloed the shark for something like 25 minutes until I timed out after everyone else in a run wiped. Shark is really nothing. Tojil's only a problem because of Terror and Break. Weakness I can deal with fine, but once I'm stoned or terror'd, that's pretty much all she wrote.
Generally speaking, though, it seems that anything DNC is capable of being healer on, it's better to just bring a WHM and let the DNC use that TP on WSes instead. At haste cap, I can get off WSes fast enough to self darkness almost perfectly reliably, assuming I can hit, and frequently can self double darkness, just from TP gained from the dual wield seizure. It's much better to bring a WHM and have me spend that TP on WSes than have to blow it on waltzes.
A DNC's healing really shines, imo, in those "oh ***" moments where the WHM is stoned or paralyzed or silenced or dead, and getting off that 1500 Curing Waltz V or 800 Divine Waltz II becomes the difference between a wipe and a little hiccough in the run.