Correct on the shorting of magic items. First thing I did was go through and add those magic items back in, as well as altering magic items to better fit the party that I had. I think that throwing some of those items in can help the party substantially.
Also note that he is basically a solo here. I think that if your group will cooperate (Aid Another, etc) to allow each of them to hit him with a "good power" each turn, he'll go down OK. Admittedly, I'm talking out my butt, since I haven't run the fight yet.
On the Plinths and the like, I was assuming that this was intentional, to either get people to use powers and abilities that allowed you to make a save when it isn't your turn, or to force people to shove their comrades out of the way. Again, since he's basically a solo in this fight, I thought that there would need to be a lot of terrain features and movement to prevent people from simply surrounding him and whacking out his hit points.
However, bottom line. Go ahead and drop his defenses if you are worried that he will be too hard, or if you are worried that the fight will be too frustrating for your group. He can always have some sort of magical minor action to boost them if you turn out to be wrong, and they start walloping him.
Players generally know how good someone's defenses are after an attack or two, so lowering them in the fight will set off people's "cheat-o-meter". On the other hand, they should have no freaking idea what powers he has, so give him a magical 'whammy' defense booster if he starts getting clobbered. Heck, make it cost hit points to use or something, so they all know why he didn't use it at the start.
Your instincts on your group are probably better than ours are, anyway.