You have to admit Creature that it is a little curious that a few players recently have posted on the forum
"I just feel like sending 15k basic troops out on a tournament square - please feel free to massacre them at your leisure."
I cant locate the original post where I read it back a while ago, but to my best recollection, SC wrote that the elemental spawn chance is (in some way) relative to the troops killed or some such.
Ergo as you yourself suggest, it is quite likely that not all combinations of elementals and units have been tried, just as there probably havent been anyone who has dared to send more than a few at a time.
What if the heart of corruption was a long standing quest that could only be solved by creating a "large" army of elementals and beat the crap out of it? Whatever rewards it might yield would be interesting - likely an evil spell - corruption of something.
Seems like a perfectly rational straight forward answer to me, but a project that may take a long time to achieve.
Illy at its best.