Off the top of my head:
If you have any 'call of nature' spell scrolls, you could try using those, as the enemy hero might target the summoned unit.
Or try moving your army around, i.e. switching the position of two or more stacks could make the enemy hero cast a different spell.
Check out these threads as well:
http://forum.1cpublishing.eu/showthread.php?t=35387
http://forum.1cpublishing.eu/showthread.php?t=36862