What where u guys doing? o.0 Never had that problem before.
Mesmers can be tricky depending on your class, I predominately play a support Guardian and Power Ranger in Spvp. I often find the best strat is to nuke down any illusions they pop up (duelist, warlock, warrior). AOE is often effective here as even buffed they have low HP and don’t move out of AOE attacks. Without their damage dealing illusions many Mesmers are almost harmless.
After that its a case of find the real Mesmer this is simply a matter of practice and trial and error. The real Mesmer will doge role and often move in a disjointed manner. Furthermore when you do find them their innately squishy so blow CD’s and nuke before they do a disappearing act.