Critical strikes in Lexandria are handled the way TSR suggests in Players Option: Spells and Magic with three exceptions.

1) A character or monster can not cause a critical strike that is higher in severity (the die roll) higher than his/her class or hit dice.*

2) A critical strike occurs when the victim rolls a natural 1 on his/her saving throw. A natural 1 or 2 rolled for the saving throw also causes a critical strike if it is a specialty of the spellcaster or monster (i.e. a red dragon's fire breath or a wizard's signature spell).

3) Only one critical strike is caused from a single attack. (It's bad enough if your fighter loses his arm to a dragon's breath, but to lose another limb or two is hard to swallow for a lot of roleplayers.)

This will prevent weak characters or monsters from causing unbalancing effects on game play.

* Spells use the level or hit dice of the caster to determine the maximum severity possible they can inflict. This applies to innate abilities as well.