In my gaming group, we don't use spell 6 in the 8 lores because most of them are broken so none of those options will work... And yeah you are right, a carnosaur would be blow away before it got close.
I think that's a brave stance to take actually. All three of those spells are very risky, they require you to spend a lot of power dice (even with a Level 4) to get it off, and if you don't IF they can be scrolled/dispelled. I don't think they are broken really, they exist to provide a hard counter to certain units (the 'unkillable monster' being the primary thing, along with Deathstar blocks). If you don't have them, you can't kill such targets effectively, and they just steamroll you. Lizardmen and High Elves are especially exposed by not having nukes or counters like 'Mindrazor', as we have no decent monstrous infantry to slay monsters with, and our infantry blocks are overcosted and underpowered.
By forbidding '6' spells, you ignore 'Pit' anyway, which is technically the best nuke the game (it's own Lore nerfs the enemy before it's cast, it doesn't backfire like 'Sun' can, and it kills monsters instead of infantry unlike 'Dwellers').