Intro:
This guide is for experienced Mesmer/Assassins. It implements a build designed to efficiently destroy a single opponent using powerful Illusion magic in combination with Daggers and some Assassin skills. It implements hexes, conditions and enchantments evenly, so countering it would be quite difficult. This build makes effective use of 3 different attributes and is very energy efficient.
Equipment:
Getting the right equipment is relatively cheap for this build. First get the best Mesmer armor available. I recommend the Charlatan's Armor, as it gives you an armor bonus while enchanted. Since this build will have you enchanted a lot, you'll make excellent use of this bonus. Get a headgear that gives +1 attribute point to Illusion Magic, and slap on a Rune of Superior Illusion Magic on it for added stat points. Place a Rune of Minor Fast Casting and a Rune of Minor Inspiration Magic on you armor, too. Also, use any Rune of Vigor and perhaps a Rune of Vitae or Attunement on the remaining armor pieces.
Next, you'll need to get a good weapon. This build is best suited to use Daggers, so you'll need a nice pair of max damage daggers with +5 Energy Mod. The other mods are pretty much worthless, so use this one. Get a Poisonous Dagger Tang so poison lasts 33% longer. Next get a Dagger Handle of Enchanting that lasts 20% longer. You should have a Poisonous Dagger of Enchanting that gives you +5 energy and does max damage. Make sure that it has a cost of 10 or less in Dagger Mastery to equip.
Attributes:
Illusion: 15 (11 + 3 + 1)
Inspiration: 11 (10 + 1)
Fast Casting: 2 (1 + 1)
Dagger Mastery: 10
Skills:

1. Golden Lotus Strike
2. Black Spider Strike
3. Illusionary Weaponry (E)
4. Conjure Phantasm
5. Fragility
6. Accumulated Pain
7. Illusion of Weakness
8. Mantra of Persistence
Golden Lotus Strike
E: 5
T: -
R: 15
If it hits, this attack strikes for +15 damage. If you are under the effects of an Enchantment, you gain 10 Energy.
This is a basic lead attack that does a bit extra damage and gives you some energy if you are enchanted, which you probably will be. The energy returned pays for this skill and the following off-hand skill. It's cheap and fast, but the long recharge prevents you from spamming this.
Black Spider Strike
E: 5
T: -
R: 12
Must strike a Hexed foe. If it hits, this attack strikes for +24 damage and target foe is Poisoned for 15 seconds.
This is a follow-up attack that does a bit more damage and then poisons the target for a substantial amount of time, plus the added bonus of 33% from that Poison Tang. This attack has to be done on a hexed foe, though, but you are a Mesmer. Just make sure the foe is hexed prior to using this skill.
Illusionary Weaponry (E)
E: 15
T: 1
R: 35
For 30 seconds, you deal no damage in melee, but whenever you attack in melee, target foe takes 40 damage.
This is a lovely elite skill that replaces your weapons will Illusionary Weapons. They can't be evaded, blocked, or reduced in damage in any way, thus supplying you with a lethal stream of constant damage. Because you have Daggers, you'll be doing this damage quite often. With a high degree in Dagger Mastery, there's a great chance you'll double-stike a few times, letting you do even more damage quickly. Finally, with your Enchanting mod on your daggers, you can keep this skill up indefinitely.
Conjure Phantasm
E: 10
T: 1
R: 5
For 14 seconds, target foe experiences -5 Health degeneration.
A must-have for any Illusionist, this skill is amazing. It only costs 10 energy to cast and it will do 140 damage over the course of 14 seconds. It casts pretty fast and only takes 5 seconds to recharge.
Fragility
E: 10
T: 1
R: 5
For 20 seconds, target foe takes 20 damage each time that foe suffers or recovers from a new condition.
This skill makes the target suffer from damage every time it gains or loses a condition. As you have two skills that give out conditions, this skill becomes exceptionally useful. Your allies can also help by putting their own conditions on the target. Unfortunately, it costs a bit of energy. The cast time and recharge aren't bad, though.
Accumulated Pain
E: 10
T: 2
R: 20
Target foe takes 35 damage. If target foe is suffering from 2 or more Hexes, that foe suffers a Deep Wound for 20 seconds.
This skill does a little damage and induces a lengthy Deep Wound on the target provided you have two hexes. Combine with Fragility for added damage and it takes up one of the two hexes required (the other being Conjure Phantasm). The energy cost, cast time, and recharge length are a bit much, but it's still worth it.
Illusion of Weakness
E: 10
T: 2
R: 30
You lose 240 health. Illusion of Weakness ends if damage drops your health below 25% of your maximum. When Illusion of Weakness ends, you gain 240 health.
Cast this before going into battle so you regain the energy and health lost in casting the spell. This spell is godly. It sets aside a portion of HP to be used when you need it most. If used and recharged fully before a battle, it gives you an extra 240 HP. The energy cost, casting time, and recharge time really aren't relevent, as this skill should be used outside of battle.
Mantra of Persistence
E: 5
T: -
R: 15
For 30 seconds, any Illusion Magic Hex you cast lasts 79% longer.
Cast this skill right before using the Conjure skills so they last even longer. The longer they last, the more damage they do. This skill is cheap, casts instantly, recharges before it runs out, and it is a stance. Stances can't be interrupted and are hard to remove.
Using Your Skills:
Battle Plan:
Before you enter battle, make sure you cast Illusion of Weakness to give yourself a strong Health buff. Wait for your lost energy and HP to regenerate before continuing. Once regenerated, cast Mantra of Persistence. This will make all your Hexes last a bit longer. Target your enemy and cast Conjure Phantasm, Fragility, then Accumulated Pain in quick precession. Charge in close and begin attacking with your Daggers. Use Golden Lotus Strike to do added damage and get more energy. Quickly follow this up with Black Spider Strike. This will poison your enemy adding another -4 to their health degeneration, as well as trigger Fragility. With the Conjure spell still in effect, they will be losing 18 HP per second for a long period of time. Once you've poisoned your target, cast Illusionairy Weaponry and revert to standard Melee attacks until the target falls, which probably won't take too long.
Vs. Individuals:
This build is definitely geared towards killing individuals, so just use the above strategy and you will cut through them light a lightsabre through melted butter. Unfortunately, you don't have much armor. The Enchantment bonus from your Charlatan's Armor and the Illusion of Weakness should help to keep you alive, but don't count on surviving too long as a tank. Just go in, get a kill, and get back to safety. Let the Warriors take the damage while you give it.
Vs. Groups:
This build isn't effective against groups. Stick to killing enemies off one at a time and leave the groups to the Nukers and Dervishes. If you must, just spam Conjure Phantasm on all the enemies. It's fairly effective.
Vs. Bosses:
Bosses are a bit harder, as hexes and conditions don't last as long against Prophecy bosses. Still, make sure you get Deep Wound and Poison on the boss. Deep Wounds really help in taking them down. Rely on Illusionary Weaponry most for dealing damage.
Combos:
Golden Lotus Strike / Black Spider Strike
This is a simple one-two Dagger Mastery combo that does a lot of damage for free. Make sure you are enchanted and that the target is hexed or this will fail. For the enchantment, you'll probably have Illusion of Weakness. The Hex will be taken care of by a Conjure spell. Start with Golden Lotus Strike. It does extra damage and will cover the cost of this combo (10 energy) from the energy boon. Follow it up with Black Spider Strike for even more damage and posion. The poison lasts extra long from the Poisonous Dagger Tang, too.
Mantra of Persistence / Conjure Phantasm / Fragility
Mantra of Persistence makes Conjure Phantasm lasts 79% longer, thus making it last for 25.06 seconds. Fragility lasts even longer at 35.8 seconds. This leaves you enough time to induce and heal both the deep wound and the poison (even with the added poison length) before the Fragility runs out.
Fragility / Accumulated Pain / Black Spider Strike
Conjure Phantasm and Fragility supply the necessary hexes to trigger the Deep Wound effect from Accumulated Pain. This effect then triggers Fragility when the Deep Wound is suffered and then recovered. Black Spider Strike also inflicts poison, triggering Fragility twice more. Both Deep Wound and Poison will run out before Fragility does, even though they last nice and long.
Illusionary Weaponry / Daggers / Dagger Mastery
Daggers have the fastest attack rate of any weapon, and Dagger Mastery gives a 2% chance of double striking per attribute point. At 10 Dagger Mastery, you'll have a 20% chance (1 in 5) of hitting twice per attack. Since Illusionary Weaponry does constant, unalterable damage, this really adds up over a short period of time.
NOTE: With Illusionary Weaponry Active, your two Assassin skills will fail, so don't use them.