>>527840515
Call it

>>95849884
There's an instant that does that. Your version should probably either counter the spell, or allow you to take control of it and choose new targets. Otherwise it just becomes a game of them naming a valid target, losing the toss, and having to repeat the process over and over again. That could easily happen 4 or more times without needing to be ridiculously unlucky.
I might also suggest that it gains counters each time it blocks a target, and sacs itself when it has enough counters.