I am afraid that you would have to discuss this with any opponents, or tournament organizers in advance, if you do not want the roll a dice rule to come into effect. I have to concur with the other lizards that indeed the Winds spell and the other plan you had would not work. If you ask my opinion though, I would be inclined to say that you test on LD 4. When someone casts Purple Sun on models without an I (even if they have a crew like Dwarven Cannons!) I remember the ruling to be that they have no I value, and are automatically slain. Same applies for when you cast Purple Sun on one of our Steggies, you use their lower I of 2 instead of the skink's I of 4.
I can remember having to use the I of 2, but now i can't find why anymore. Seems i wasn't quite right!