(edited by Jonathan Schelling.8106)
Mesmers and WvW zerg
Play a different class.
Mesmers in zergs are either veil/portal bots or backline snipers.
https://www.youtube.com/channel/UCJTjZ2yriM20o_36MZmMTuA/videos
The proper answer is to wait for HoT, when we get Chronomancer. In the meantime though, there are some things you can do that I’ve had success with.
I tend to run gs/staff with Dom/duel/chaos, taking every single interrupt trait. I take MoD and decoy, then sometimes feedback and sometimes blink.
The way you want to play this is dropping chaos storm with the mantra on regroup manouvers. They generally won’t have stab up then, and they’ll be using longer channeled skills like empower and such. Loading interrupts into that will not only stop the empowers from executing, but will do damage, immob, and spread significant confusion onto everyone nearby (enough for it to hurt even without condition damage).
Alternatively, you could play a staff ele and do far more damage with far less effort, but that choice is up to you.