I play thief and mesmer, and like battling each with the other (does that make sense?). You have a few things wrong here, so I’ll explain them:
1 – clones (the exact copies) can apply conditions on death, if the mesmer has the right traits. Phantasms can’t.
2 – mesmers have access to 4 shatters including a daze (probably what you experienced when hitting phantasms) -that is on a long cooldown, and distortion, which makes them go all blurry and gives them evasion for 1 second per illusion shattered. Shatters are different than clone death, so if clones are causing conditions on death,the mesmer will probably not be shattering much.
I think the first thing you need to do vs a mesmer is slow down. Don’t panic, and remember to keep the fight in your control. Mesmers do have access to stealth, but their stealth is much more limited than thief stealth – so you can always wait them out. If you have dagger off-hand, you can delay the fight indefinitely, and only fight on your terms. In fact, the mesmer makes this easier than most classes, as you can cloak and dagger off of any illusion (without worrying about dodges). Just be patient and look for an opening.
The next thing to do is recognize what type of mesmer you are fighting – if they have any signets, those will be a tell. Signet of domination is either one of the very rare shutdown type builds, or more commonly a condition build (yay, thieves have good condition clears), signet of illusions is probably a phantasm build. If there are no signets, then pay attention to play-style. Are they summoning a lot of phantasms? Are they dodging and leaving clones behind? The most important distinction to make is shatter vs phantasm build.
For Phantasm builds – the phantasms hit like trucks, and they will hit you when you come out of stealth. So you want to get your damage in on the mesmer, and focus on dodging phantasms. Also, if there is no signet of illusions, then the phantasms are fairly easy to kill, so if you can take them out with a quick autoattack, an aoe skill or CnD, then that’s good. Don’t waste too much initiative on phantasms though, the mesmer can bring them back about every 20 seconds or so (and will usually have 2-3 every 20 seconds in a phantasm build). Also recognize that phantasm mesmers are often considered OP in 1v1 battles (they’re average in small groups and ok in zerg v zerg). On the other hand, thieves can win 1v1s against them if they play well…you just need to practice it.
For shatter builds – the shatters are the main source of damage, and damage comes in bursts. The biggest tell is a mesmer dodging for no reason other than to create a clone. Even having that trait suggests a shatter build (except for a few condition builds). The important thing here is to watch for tells that a burst is coming. When a mesmer shatters, their illusions all rush towards a target at once and then put their hands to their sides and look up- then turn into butterflies. You can run away from the clones, and you can dodge the shatter effects. Because of that, mesmers often use combos that set up a shatter. One burst is illusionary leap – the mesmer, wielding a 1 handed sword – sends a clone leaping toward you – which cripples, then swaps places, which immobilizes, then blurred frenzies you, while shattering. If you see a mesmer with a one handed sword, save a dodge for that initial clone that jumps toward you – and you may avoid the burst. There are also mini-bursts – like a greatsword mesmer might send out a phantasm, then a clone then shatter. You can avoid this by cloak and daggering off the phantasm or clone as soon as you see it. Basically look for a sign that the shatter is coming and avoid it, and you can beat a shatter mesmer.
In either case, the most important thing to do is not waste too much initiative on illusions, and use illusions to keep you stealthed until you are ready to hit the real mesmer.
One more thing – you may want to ask a similar question on the mesmer forums (be polite and don’t tell them they’re overpowered), they will probably be very helpful.
