Shield Bash and interrupt.
http://wiki.guildwars2.com/wiki/Defiant
http://wiki.guildwars2.com/wiki/Unshakable
Basically every time a creature with unshakable gets CC’d (ie basically every boss and some champions) they gain a stack of defiant for every player in the area. Each of these defiant stacks stops 1 CC effect. So in a 5man party basically only every 6th CC used against the boss will work