[Excist]

1) Usually DRN's are opposed rolls (both sides get a roll and then results are compared) and also usually open ended (all 6's add a bonus roll to the total) which makes odds fairly predictable, but leaving all possibilities...well, possible.

Magic Duel (single opposed single opposed roll) being one of the rare exceptions.

4) Spell effects do not stack with each other, but spell effects do stack with magic item and innate bonuses (which also stack with each other)

6) Creativity. Putting bodyguards on your mages with equipment or spells geared towards immobilizing or killing the storm immune flyer that has been plaguing you would be a thing to try.