Base the spell on Berserk with 0.01 seconds duration. Add a 100% evasion ability inside a spell book. By using triggers, add the spell book to the caster when the base Berserk spell is used. After 0.5 seconds, remove the spell book.
I would rather do what _Famous suggested, but the method Glenphir suggested hides the evasion icon (I think), so long as you hide the spellbook (an option in the ability editor).
Making spellbook makes it unlevelable, if you disable it. If you add the item ability, and the hero just nicely level up, he might waste a point on it.
nono its good .. well explained .. but there's no need to use spellbook if u can add evasion itself oO
Just to throw another suggestion, instead of Berserk use Wind Walk and set the Transition Time to be longer than the duration, then your hero will automatically become transparent upon casting.
This is also possible through triggers, but I thought Wind Walk would base a better base ability for this.