I've touched upon this before, and inspired by the recent trends in the army book releases, we've narrowed down our potential ways to combat this problem to two:
Spells such as Purple Sun, Pit of Shades and Dwellers Below, instead of automatically slaying a model if they fail their test, will cause wounds equal to the amount by which the model fail their test by, with no armor saves allowed. The Curse of the Horned Rat will cause the same 4D6 wounds but with no armor saves allowed (and its special rule still intact). The last sentence of the Gateway spell (a strength roll of 11-12 automatically removes a unit) will be ignored.
The idea here is to make it possible to somewhat limit the awesome effects of these few spells, that many feel are ruining the game. While they will still be extremely lethal for most units, it is possible to at least limit the damage they cause. Magic Resistance for added ward save against such spells would then be worth taking. Ogres and Trolls will still be very much vulnerable to the initiative tests, but one Purple Sun is unlikely to ruin your entire battleline (although it will still hurt a lot). Also, for those who are used to losing your Archmage in round 1 to an IF Dwellers Below, now, even if you do happen to roll 6 for that S3 test, you'll at least get your ward save. Warmachines and other devices with no initiative value, will by the same idea automatically suffer wounds equal to what they fail their test by (so in effect take D6 wounds).
Personally I feel this is a good solution that would allow the spells to retain their lethal touch, while preventing them from being game breaking and ending a match in a single round of a lucky 6-dice spell.
The second solution mentioned is to lower the maximum amount of dice you're allowed to cast at a spell to 5, from 6. This would mean that for those heavy spells that require 25+ to cast, you'll have to roll exactly average for your level 4 wizard to succeed. Also, for the defender, if you're lucky you'll have 6-7 dice of your own to dispel with, which increases your chances by quite a lot.
I believe this solution could work as well, although I fear it won't do enough to combat the problem, which is that the magic phase only consists of players taking all their power dice and tossing it at their 6th spell, because if you can get it through you'll effectively win the game in many cases.
Either way, I'd love to hear your thoughts on this matter, as I feel expanding the discussion from just our local players to other people around the world can only do the matter some good.