By the sounds of it you are dealing with High Burst Dps Assasins, so im asuming and this is only an asumption that they wont be tank spec and this difference is vital. The thing you have to remember is these Hight burst Dps Shadow/Assasin specs are just as squishy as you...if not more so as they do not have static barrier, so as scary as this may seem sometimes as you try to flee or kite, you better off turning round and to put it in simple terms having a dps race to see if you can nuke them 1st. Its a given your not always going to come out on top and pvp would be boring if you did .
And about you comment that all classes have a class there weaker against, the fact that every class has one makes it balanced surely instead of a class that has no weakness.
Hope you have better look in the future but some times its not a case of Balance or that over used "OP" but a case of learning how to fight different classes GL in future warzones.