One quick bit of advice: split the party. Not necessarily into multiple groups with separate encounters (unless they're really big, of course) -- but if you use encounter designs that naturally split the party into multiple groups, well, super tank can't be -everywhere-. Put the NPC artillery on one end of the room, a NPC they ahve to save an another, and the controls for a painful trap somewhere in the middle, and make them choose how to deal with all of these simultaneously (or stick in one big lump and take the consequences).