Myr Superion has an ability that requires all mana that is spent to play it be generated by creature sources. However, it is possible to bypass this restriction by lowering the cost of the Superion enough that no mana is paid to cast it. The ability basically means, “You may not spend mana from a noncreature source to cast Myr Superion.”

Heartless Summoning lowers the cost to cast any creature spell by two generic mana. As the Myr Superion only costs two mana, this will lower its cost to zero. It can now be played without any mana being spent. The Superion’s ability will not prevent this from happening, as there is not mana from a noncreature source being spent to cast the Superion. (Just remember the Myr Superion will enter as a 4/5 due to Heartless Summoning’s other ability.)