Yeah, just allocate shifts to power and duration, brew 'em up, and you're ready to roll. I'd think it would work just like a normal block: every shift of power increases it by one point, every shift of persistence adds another exchange, so it would cost you 10 shifts to set up a Fantastic (+6) block lasting four exchanges.
Of course, to target a specific caster you'd need your symbolic link. IIRC, Harry had something belonging to Kravos when he shut his magic down. I think the reason Eb could do without when locking down Mavra is that undead magic is substantially different than mortal magic, and he was able to shut down Blampire spellcasting without affecting Harry, though that's just my theory.