Only fight without a cast bar I've managed to get it right every time so far is beginning of the second phase in Shinryu EX.
It does have a castbar. Look at enmity list.

Aggro dump : YES PLEASE. It always frustrates me when PUG co-tank refuses for some reason to circle-Shirk, as our SAM enmity always creeps way too fast and we have no NIN.