Cripples the next creep that leaks so that it does 1/2 as much damage to your wizard's health.
Maybe make it do none at all. I'm unsure about allowing this to be a guaranteed safe blanket for blood demon users.
This feels like it should be an uncommon card. Perhaps just give 1/3 damage protection, i.e. 2/3 damage taken.
Wizard Guardian (prophecy)
A slightly faulty shield, but prophesied to appear with perfect timing.
Reduces damage done to your wizard by 1/3 next time a creep leaks.
--- or ---
Timely Racquet (prophesy)
This racquet has been cursed to travel through time in a reverse. It is destined to appear at the most opportune moment.
Next creep that tries to reach your wizard's tower is swatted back to the middle of the map. The swatted creep takes 10% damage.