Walkthroughs for custom maps.

Disrupting Ray is one of my favorite spells (of course, it doesn't always play a decisive role, but sometimes it's with its help that difficult battles end). Here is a complete description from PhysMag 2.53:

It can be cast multiple times. It is not removed by Healing or Spell Removal. After a unit is resurrected, the effect of Disrupting Ray, applied before death, persists.

In addition, it is not mentioned there that Disrupting Ray does not have a duration that depends on the strength of magic: once cast, it remains effective until the end of the battle. So, it's a "tool against which there is no defense."