What I am thinking now as more accurate would be that Xhadar could get close enough once every 3 beats provided he spent the intervening beats moving 2 steps. He would still need to overcome a difficulty of 2 or 3 to hit with his throw, regardless of any defense Ser made.
Maybe I shouldn't say this, but for the sake of the discussion, this is what I would do if I was controlling Ser.
-Take the 2 free attacks while moving backwards.
-When Xhadar gets close enough to attack, flare steel and try to push away the weights he is carrying. Then Xhadar has a problem, if he saves his dice for attacking he has a 99% chance of losing the weights, so he has to defend. If he puts all his dice into defense to give him a roughly even chance of holding onto the weights he won't get the opportunity to attack. His only real option then is to split his dice 50/50 and try to defend against 8 dice with 4 or less, a possible bet, but a low chance of success, then use the remaining 3 or 4 dice to try to beat the difficulty and hit Ser if he still has the weights, still not a great chance but possible.
The other important point is that his plate has already taken 3 damage, if it takes another 2 points of damage it won't be enhancing his strength anymore and not only will he have a grand total of 5 or 6 dice to defend and attack with, without his plate enhancing his strength he would need to get closer to hit with a weight, so the difficulty of hitting would go up and/or he would need to move closer, making it once every 4 beats that he got a chance to attack.
Things would be very different if he had taken a bow and flint arrows instead of a grand bow but it didn't make sense for him to know to do that
I think 2 thugs take a full shardbearer would be an interesting match. It would all hinge on the shardbearer taking down one of the thugs before he took 5 or more hits to his plate because at that point while he still has the defense of the plate, the remaining thug will have twice the dice that he does making hitting quite unlikely. The thugs huge dice pools also allow them to score a lot of nudges to beat the plates armour value. In the opening rounds the shardbearer would have little choice but to commit to full attack against one of the shardbearers while the other whaled on him, if he tried to split his dice for defense he would most likely just fail both rolls. *shrug* he could win, but he might not too.
From what we have seen in this fight, yeah, shardbearerse are crazy powerful against close quarters enemies, though still not close to as powerful as they are in the books. They draw a very nice parrallel with Keepers actually. A prepared keeper is nigh on unstoppable in close quarters combat, but recharging the metal minds can take a significant period of time and they aren't terribly useful at ranged combat without a bow and flint arrows. Similarly a shardbearer is enormously powerful at CQC when prepared, but as their armour cracks they become progressively more useless and the cost in regrowing shardplate is massive.
In a similar sense, properly prepared mistborn with atium are almost untouchable, not because they can take or heal damage, but because if the setup is right they can control the battlefield and prevent anyone from having a chance to attack them, there are ways to work around their defense: flint arrows for example, but like keepers and shardbearers they are enormously powerful in the right situations.
With any rules for MAG there has to be a balance point between making the game balanced and making the powers behave as they are supposed to. As it is we have all noticed that an unpowered character is weaker than a powered character and a misting is basically always weaker than a mistborn or keeper. (It does depend on circumstances of course).