I voted it is balanced, if it protects against sneak attacks.
1)
The duration is quite short, and so no spellcaster will have "activated" it during an ambush, and if he is "sneaked" then, it will most possibily outright kill him.

(And rogues planning an ambush might know who can cast the spells, and will prefer killing him because you never know what he might cast at you - and he is the easist one to kill fast)
2)
If it didn`t work against sneak attacks, then there would be no reason for a spellcaster to cast it during a battle - he might better cast a cat`s grace, or a Mage Armor, or an Acid Arrow, or whatever he might have available. Criticals happen to seldom.
3)
The spell is a bit like "See Invisibility". If you know there wil be enemies using Invisibility, you will memorize it, if you don`t expect them, you will rely on a scroll (or perhaps even a wand.

)
If you expect to come under attack by rogues/assasins, than you will memorize the spell.
4)
The spell description seems to indicate (just by the flavor text), that you lose any vital organs, and that is the place where sneak attacks strike...
Two Things I would change:
a) Since it is Transmutation, and it changes your body, it should be:a Fortitude (Harmless) Save.
b) Range Touch would be okay, I think, but if you feel uncertain about it, it would be okay to make it a 3rd level spell.
For kreynolds idea of making it a % chance: (Caster Level) x 10% might be a good idea.
For a higher level spell: It could be a mass spell (bumping it at least for 3 levels, example is Haste -> Mass Haste), or it might have a longer duration. (Minutes, Hours, or even a single day - the last one is a good idea for high ranking nobles, but it would impact the setting - how was the assasin able to sneak and death attack the king, if this spell was available?)