How can I guard-break uruks with iron shield?

For most Uruks, you can just vault over. But there are some Uruks (especially Warchiefs) that have a strength that makes it impossible for you to vault over. What you can try then, is use your Wraith Stun (E) on them. This incapacitates them for a little while, enough time to vault over them, or use Wraith Flurry to do some damage.

If that doesn't work, you have your combat finishers. Once you get to the number of combo points you need for a finisher, just use it on the Uruk in question. When it's done, you can use normal attacks to hit him even more.

You can also try to use ranged attacks, if the Uruk in question is not immune to ranged attacks. Use one or two arrows, and continue with your sword.

If there is a Caragor at your disposal, you can also just use mounted attacks against the Uruk in question.

I hope these tips help. Enjoy!

I once had to face a defender captain that was immune to most normal tactics. In the end I was able to isolate him outside of a fortress, kill his companions, and kill him (slowly) with throwing daggers. I've not really mastered the shoulder slam maneuver yet but that may also be of value.