I am not a Zerk player but I find it quite ok as it is. The damage of his lights (and heavies) is a bit delicate in my opinion and could be toned down a little (as for a lot of other assassins too). He has no openers and thus has to rely on his feint game (which is quite good imho), aggression and whiffs and his hyperarmor.
The question at hand is what exactly is your problem there? Do you fall for these feints and tend to throw out a parry attempt? Well in that case I think you just need to get a little bit more used to what Zerks do. I've been there too, but there is also just so much a Zerk can do (literally nothing) when you sit there for a moment and block. Also, most zerks tend to go for a certain pattern all the time