Its a very logical requirement. Activating hero flash at 10th turn lets you summon a normal hero from your deck. It would be nice if there still were normal hero monsters in your deck at that moment. Very easy requirement to meet.
Pretty oddball skill and mostly just a gimmick, it does add cards for free but also requires running 4 DIFFERENTLY NAMEd normal E Heroes. Best used alongside some copies of Neos together with Grandmerge.