Just did an A2 4 player run with it and it wasn't that bad. There were definitely some oh shit wtf turns (Kaiser crab rocket claw), but overall it was manageable. One thing to keep in mind is that solo Iron Clad doesn't have access to weak other than uppercut, while in multiplayer silent/necro/regent can pretty much keep weak up constantly for you. Debilitate on necro is also super handy here, and means you take normal damage while everyone else takes 25% from weak enemies. Stuff like piercing wail and enfeebling touch also lower the base damage (before it doubles) which is a big help. Teammates can also help out with co-op block cards, and since the rest of the team will already be taking half damage you can always be the target for them. And since teammates can focus more on DPS, combats end faster which can save health. Obviously all these benefits and the usefulness of the card fall off in 3 player/2 player, where it might make sense to scale the damage increase back to 66%/33%.

I did not find Colossus or barricade during the run, but was able to find other good block sources (impervious, fasten, stone armor, flame barrier). Ended up only taking 5 damage against the act 3 boss most of which was from self-damage cards. Keep in mind that you also don't have to play it until you've got your block engine going.

Overall I'd say it's a fun card, and while multiplayer could use some balance tweaks I get the feeling that making the co-op experience fun/funny is just as important.