12 shield skill = hard to break shield
13 shield skill = unbreakable shield
13 shield skill leaves room for... pretty much nothing else. They're pretty fast on their feet because athletics still gives a slight bonus to runspeed. Something you will notice when you have 39 of it.
But if you do encounter such a player, just bring a crushthrough weapon. Even if you don't have any prof in 2h or polearms, it's not that hard to bring one and whack em over the head with it.
Crossbows still penetrate the shields.
You can still kick/slash them
Attacking them with more than one player and NOT just attack to try and break their shield will get them killed too. They can't face all directions at once.
I've played around with a 13 shield skill shielder when the rank 10 cap was first removed and it's not nearly as powerful as you might think.