I don’t have any experience with Engineers (unless you count Level 5 as having experience), so I’ll ignore them for now.
My main character is a Mesmer, so I’ll try and explain what I’ve come to learn about them. That said, my detailing may be a bit more… theatrical than necessary.
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Mesmers are the illusionists of the game. They’re the ones that like to create a show. They use misdirection and punishment to keep you at bay, all while barely even touching you themselves.
You have access to Illusions; your Clones and Phantasms.
- Clones are the illusions that look just like you. Their attacks deal no (or very meager amounts of) damage, but they often have some sort of extra effect. Staff clones, for example, use Winds of Chaos, an attack that bounces a couple times between foes and allies applying conditions and boons, respectively. While they lack any real damage with their attacks, they’re the sturdier and usually more readily available of the two types of illusions.
- Phantasms are the illusions that look like a purple-ish ghostly version of you. Their attacks, like Clones, have extra (and normally more powerful) effects, but they can also actually deal damage as well. The Focus phantasm, Phantasmal Warden, sits at the target, and whirls his weapon around in the air for a few seconds that not only damages the target, but also reflects projectiles and even acts as a Whirl Combo Finisher (combining well with your multitude of Ethereal Combo Fields to shoot Confounding Bolts [inflicts Confusion to foes hit] or Focus #4 – Temporal Curtain, a Light Combo Field, to shoot Cleansing Bolts [removes conditions to allies hit]). While Phantasms attack with greater potency, they’re not very durable, so they’re generally only around to deal one attack before being shattered or destroyed.
Your Illusions are used to support yourself and hinder foes, help take/keep the focus away from yourself, and fueling Shatters that can apply effects or purely deal damage and increase strength the more Illusions Shattered.
Mesmers have ways to evade or nullify hits via leaps, blocks, reflects, and teleports and a myriad of ways to punish their foes for even trying. Chaos Storm creates an area that, while standing inside, grants boons to allies and conditions to foes for being hit. Even their primary condition, Confusion, follows that same principle.
Mesmers excel against small groups. They do well in prolonged fights, gaining the advantage the longer the battle lasts. The strength of a Mesmer is their use of misdirection and punishment.
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Unfortunately, that’s also their biggest weakness. Smoke and mirrors work at certain angles, confusing and bewildering those that see from that side. Get a ton of eyes looking at you from all directions, and the illusion can easily fall apart. Of course, seeing it without the set up can render the illusion useless as well.
Rather, a Mesmer can generally handle a few foes at a time (the illusion works from specific angles), but a large mob of enemies can easily overwhelm them (the illusion breaks from everywhere else). Just the same, they can handle taking damage over the course of a prolonged fight (the illusion is being built up), but a sudden burst is not easily survived (the illusion was never set up).
The same can be said about PvP and WvWvW. Against one or even two people, you can pretty much handle yourself just fine. Any more come after you, and you better hightail it out of there quickly. With that, you’re great if you can keep the battle going, but you’re pretty much dead if someone tries to burst you down.
tl;dr
Mesmers fight using misdirection and punishment, favoring prolonged battles against small groups, but can be easily overwhelmed when focused or burst. I like using metaphors to explain things. I’m unable to give a plain and simple explanation.
