There is no argument that for a single boss a mage should use the Dragon Staff for the % HP drain. There are some mages who confuse the charged attack (Consecration, which is only a buff/AOE) and spam that instead of the normal attack which does the proc.

+1 on using Immortal Staff for mobs. While I still have a hexstone staff and watching Wildfire burn everything is still super powerful and fun, I prefer the fast banish when you're in a PT with good complementary AOE procs, worm/SNS pools etc. it's just underrated because Wildfire looks so impressive, while Rotting Flesh just made chunks of enemies disappear with little commotion . The higher the health of the enemy the better chance at hitting enough Rotting Flesh stacks to cause the banish, so it'll be even better in M5 as stated by others.