Versus Advanced Iron Legacy prioritize:
Redirection: The best way to get through his defenses is have him hit himself.
Prevention/Immunity: If he can't hurt you it doesn't matter if you can't reduce the damage.
Ongoing Destruction: Need this, more is better.
Damage and Healing Bursts: slow and steady loses the race.
Card prevention: he still deals damage, but you save yourself a lot of nastiness.
Card spamming: Lastly IL has no all destructs, he destroys your stuff in small bursts, get ahead of his destruction and you can beat him.
So your top heroes are:
#1: Nightmist. The whole spectrum, redirect, stop his cards, big healing, destruction and immunity. Whe doesn't have huge damage, but if you can get your cards before you die you should win. She needs friends that put ongoing and equips in play to save hers from destruction.
#2: Tempest. Awesome Destruction, great damage, and his incap is glorious when he falls. if a damage dealer with some healing is alive when Tempest falls it is likely game over for Iron Legacy.
#3: Legacy. Take Down, Interception, Lead+Evolution, Galvanize, it is all glorious. No destruction warning!
#4: Haka. Ground Pound, Haka of Battle, more health than IL. No destruction!
#5: Mr. Fixer. Yeah, that's right. No destruction! Tiger/Crane/Mantis are all good vs. IL. And Mantis is Huge when you have an ally that hits multiple targets of their choice. He also has Grease Gun. If he isn't doing well he still puts cards in play to eat destruction which is big, and Salvage yard can be a game changer.
#6: Fanatic. End of Days, Wrathful, Aegis, focus and Sacrosanct Martyr. Your health is irrelevant, Iron Legacy's health is all that matters. Fanatic lives for short brutal fights like this.
#7: Parse. Destruction, Irreducible damage galore, and Buffer Overflow can put his indestructible cards in the trash. Parse and Fixer are a fantastic combo vs. IL. Two rounds of irreducible damage is a win with almost any team.
#8: Wraith. Whe gets in front of destruction, she deals solid damage, she has throat Punch and Grappling Hook, Smoke Bombs is the bomb with Nightmist if she gets Amulet and you can feed Nightmist enough cards. Round 2 Iron Legacy wiping himself out from nearly full health is priceless.
They aren't in order, just your best options for building a team around, in my opinion.
Don't roll with 2 of Haka/Legacy/Fixer unless you go 5 heroes, and even then you are asking for trouble.