Asura.Pergatory said:
Definitely use WHM/SCH for Impact casting. The elemental skill from Dark Arts is vital, and you can use Parsimony to cut the cost down considerably.
I use a build similar to the one you posted and it works well. Even with the skill from Dark Arts, I still sometimes see resists. When I try to cast it on my SMN (which is pretty much never /SCH) it's almost always resisted down to 100-200 damage and about 10 seconds of attribute down which is pointless. I imagine it'd be the same on WHM without Dark Arts.
Honestly though, you should be using /SCH pretty much all the time anyway.
This, On my decked out blm I only reach between 4~5K depending on the nm for 666 mp so its really not that efficient but! Impact really shines as an elemental debuff, -20% to every stat so in other words. More dmg to the mob, less dmg dealt from the mob, and my favorite, less resist for those people that cant land procs.
I often use it on a nm if someone is having trouble landing a yellow proc or if its just rape time on the nm. So my suggestion would be the same as Pergatory, /sch and get your elemental as high as you can so the spell goes unresisted and last the full duration which i believe to be around 2mins+ but am unsure.