Banish spells use the exact same formula as the elemental nukes, except they use dMND instead of dINT. Actually you should be thankfuly, the original Banish spells used dINT, MND did nothing for them. SE basically took Aero and put a long recast with "light" based damage. That whole "undead" thing was added later when SE decided to make them use dMND instead of dINT.

Anyhow, the reason most WHM's banish's seem to small is WHM's normally don't carry around MAB gear nor equip +Light Damage / MAB atma's. Otherwise Banish III does comparable damage to Stone III
http://wiki.ffxiclopedia.org/wiki/Ca...g_Magic_Damage
Stone III
92mp
Base V of 210
M multiplier of 1.5

Banish III
96mp
M multiplier of 1.5
Base V of 198
Now saying this, I fully believe WHM's should have Banish IV, V, Banishga III, Banishja and Holy II. You guys are supposed to be the kings of divine based light damage, its not used often but it's something that does exist, the holy nukes shouldn't end at tier III.

Basically as a WHM/RDM with HQ Light staff, Beyond / Allure / Holy Mountain, and tons of MND / MAB gear, you should be hitting big Banish IIIs. The utility of this is questionable but it ~is~ possible, there is nothing with WHM or Banish that prevents it from doing high damage. SE just needs to add bigger spells for christs sake.