To save WHMs some sanity before big boss fights:

/ac "Stoneskin" <1>
/wait 4*
/ac "Stoneskin" <2>
/wait 4
/ac "Stoneskin" <3>
/wait 4
/ac "Stoneskin" <4>
etc. etc.

There's 15 lines maximum to a macro so you can fit all party members in there.

* Even though wait 4 might seem long considering Stoneskin's casting time, the macro seems to be rather hit and miss when using wait 3, often missing every other cast.