I'm kiling these things as a 75RDM/WHM and I'm having no problem with them.
Stoneskin macro:
/ma "Stoneskin" <me>
/wait 8
/ma Blink <me>
/wait 11
/ma Phalanx <me>
As well as macros for: Refresh, Cure III, Cure IV, Enfire, Blaze Spikes, Burning Blade, Shining Blade, Fire II & Fire III.
They are casting Dimensional Death, but with SS, Blink & Phalanx up, they're hitting for 0. They only times it gets me (usually for 50-90) is right as SS wears. The Black Mage casted Stonega II or III (can't remember which, lol), but hit for 0 with SS macro ^_^ I highly suggest keeping his silenced, however! I noticed as soon as he starts casting, if I cast silence, it'll interupt his casting.
Hope this was informative and helpful, and happy killing! LOLz