It would be much easier for us to say what made them hard as steel if you would tell us wich champs we are talking about.
Some suggestions: High defense, passive effects like Ursine Icecrusher's Shroud of Winter (decreases the damage all allies receive from critical hits by 15%), Stalwart set...
Some players do this... just because a team has just one opponent doesnt mean its an easy win. If you put super end game gear on a rare champ they will basically become legendary. It's been said many many times.... this game like most is all about the artifacts, not so much the champ that is wearing them.