Maybe I a bit overstated the percentages, but from my own experience against really good 2 handers it ain't that simple, since most of the 2 handers has agi based builds so they can simply outrun/outrange you, as you now shielder has rather heavy eq what makes him vulnerable to whose kind of oponenet movements. So it's not about breaking your shield. Avarage player with greatsword/longsword in duel can hitslash you even if you hold your shield, this is happening to me very often. What is more I'seen a lot of this kind of fights, and in most of them 2 handers won... unless they can't block properly. There is one more "dirty trick" that gives a huge advantage against shielders, kick slashing... but to be honest only a few players in EU keeps using it. With shield this technique is way more harder, if you'll miss be sure that you get hitted.
Of course if you're a decent shielder it won't be that easy to kill you, but it seems that Myltinous, to whom I was talking to, isn't any experienced in fighting with shield.