How do you win against a phantasm mesmer ?
Dodge/reflect the phantasm. Throw a random attack at the paper phantasm to kill it while focusing the Mesmer.
Zzz.
Not as easy as it sounds. While you are trying to harass Phantasms (it will take more than 1 attack unless you are specced VERY heavily glass cannon or blow a CD on it usually, from my experience) the Mesmer is free to do what he wants. Maybe he’ll interrupt you trying to kill his Phantasm. Maybe he’ll summon another one, maybe he’ll Illusionary Leap + Swap + Blurred Frenzy when you get close (assuming in melee) so the Phantasm can do whatever it wants while you take some DPS. Maybe the Mesmer will run away.
You could also kill him, if he let’s you I guess.
Berserker phantasms are a bit of a pain. Duelist phantasms are best killed if you can put up some kind of reflect. The others are pretty safe to ignore (unless you are chock-full-o-conditions).
you dont
But more seriously, do kill phantasms (dont focus them, but do use aoe and chain moves to hit both mesmer and the phantasms), dont kill clones (let them die to incidental aoe). make sure you dont stand in fields or with fields between you and phantasms (duellist) otherwise you will be combo’d.
Some phantasms have very slow attack speeds, berserker for instance, you can dodge it really easy, as it just stands there until it spins past, so move away from it if possible to give yourself time.
Look out for confusion, learn to spot it and react to it.
Almost all classes are faster than mesmers if they take speed traits, use that advantage, esp if you are ranged.