This is my WvW roaming build. It is made for solo and small group fights, getting behind enemy lines, stealing camps, disabling siege and orbiting zergs. It is a power shatter build that relies on Prismatic Understanding (PU) and stealth. It is good for both dueling and assasination, but does not excell at either. It does well against projectile heavy builds, zerg builds and bad players. It is weak against reveal, heavy aoe pressure and heavy condition pressure. Most professions (except perhaps Necro and Ele) have roaming builds available that can counter this build, but they need to be played by experienced players. Good Eles can outsustian this build. Necros are just screwed.
Greatsword is the main weapon for power shatter Mesmer. Traited it has decent dps because of the reduced cooldowns on #2 and #4, and every skill (except #1) cripples the target.
I prefer scepter over sword on this build, mainly because its defensive skill (#2) doesn't root you in place, and because it has some range. #2 - block - F1 is also a good combo, but you should generally only F1 when on greatsword, because it does slightly more damage.
As this is a stealth based build torch is probably the best offhand weapon. The #5 skill isn't very good though because the phantasm's attack rarely connects. For clearing camps use a focus instead and pull the guards together, then burst combo them. A focus is also a good weapon for pulling enemies from walls and when you run in a zerg.
Domination 1 1 1
Domination is not a very good trait line but I think it works better than Illusions for a power build.
Confounding Suggestions is nice for a F3-F1 shatter burst and the occassional up to 4 seconds chain stun.
Shattered Concentration for boon removal, not unimportant in the current meta. Furious Interruption can be an alternative, but this build is not based on interrupts and with the amount of stability many classes have in WvW, interrupt builds are more difficult to play. In certain situations FI can work well too, though.
Imagined Burden for better clone generation and generally increased dps. Also the cripple it provides is often very usefull in roaming situations, for kiting and for chasing.
Dueling 1 2 1
Dueling is a must.
Phantasmal Fury, because this build doesn't use pistol and Desparate Decoy is not very useful, since it interrupts skill and if you don't pay attention you will just immediatly un-stealth yourself. Other professions have better passive defences.
Evasive Mirror over Blinding Dissipation because of the many pew pews. If you encounter many warriors and thieves you want to switch over for the blinds.
Superiority Complex to help with bursts and general dps, as power Mesmer dps isn't all that great. Against certain builds it can be usefull to switch to Deceptive Evasion instead, but core Mesmer can't benefit from that in the same way Mirage can.
Chaos 3 1 2
This is what really defines this build. Chaos turns this into something better than a one-shot or die meme build, with its access to defensive boons.
Master of Manipulation gives this Manipulation heavy build the edge. Reduced cooldown and superspeed make all the difference in the world.
Auspicious Anguish has great defensive benefits. Not only F4, but also becoming disabled will trigger it and it has no internal cooldown. This is a life saver on a build that is otherwise weak against condition damage.
Prismatic Understanding commits you to a stealth heavy build, with all its advantages and disadvantages. It helps you to get in and get out and provides boons.
Using Master of Manipulation Mirror becomes the heal of choice. It reduces the cooldown to 12 seconds and adds 3 seconds of super speed. Basically you have a on demand 2 second reflect on a 12 second cool down that also heals you and gives you superspeed.
Blink is a given even without Master of Manipulation. Traited its cooldown is reduced to 24 seconds and it also gets superspeed, for increased kiting/chasing potential.
Decoy, because it's a PU build. A usefull alternative for that is Acrane Thievery, which is a Manipulation and generally a very usefull trait. It depends on the situation which one is better.
Signet of Midnight. This is an easy choice in a PU build. It's a great stomp helper with its stealth and blind and unlike Decoy it doesn't interrupt stomp. The passive is also usefull, even on a power build, think about vulnerability, blind and cripple. When running in a zerg I usually replace it with veil.
For the elite there is no other choice than Mass Invisibility. And it is really great, traited it has a 48 second cooldown and lasts 9 seconds, and has 3 seconds of superspeed.
Armor and stuff:
Basically any combination of berserker and marauder that you like. Personally I like to have about 2500 power and 16000 - 17000 hitpoints.
Runes: I use Rune of Fireworks, because it's the rune that gives passive movement speed that works best for a power build. The boon duration is usefull and power, fury and vigor infight help too. Ideally I would use something like scholars, but I just cannot live with core Mesmer's slow base movement speed. I also feel like speed is important for roaming.
Sigil of Energy on both sets. Even though this is not a mirage build I believe the ability to dodge is so important in so many situations.
Sigil of Celerity on greatsword, the 5 seconds of quickness on disable work very well in a F3 - F1 shatter combo, or after Illusionary Wave. If you face a lot of condition heavy pressure think about replacing it with a Sigil of Cleansing.
Sigil of Cleansing on the other set.
Orrian Truffle and Meat Stew for even more dodges, Superior Sharpening Stones for a bit more damage.
This is not always easy to play, you have to be able to do the basic Mesmer burst shatter combos. At the same time it gives you some options if something goes wrong. Use your stealth to get out of a difficult situation, or to set up a burst combo. You can get up to 21 seconds of continous stealth (even more if you use Mimic), but Mass Invisibility is usually enough to sneak up on an unsuspecting victim. Use the superspeed on your heal and on Blink to kite warriors until you can burst them down. Be ready to use your reflects on all kinds of pew pews, then punish them. Don't get dragged into unwinable situations (like I usually do), you have stealth and some mobility, often you are the one who decides when or when not to fight. You can be a very annoying siege disabler, the cooldowns on Mass Invisibility and Siege Disabler are very similar. Stack stealth, move into position and wait for all the bubbles to go off, then throw the disabler and blink away. Rinse and repeat. If you make them use stealth traps you know you did a good job.
Core Mesmer isn't in a great spot right now, so good players on many builds will be able to force a draw or just outright kill you. Thief is a hard counter on most builds, warrior as well. Playing against these it is their fight to lose. Certain Soulbeast builds (the not so pew pew ones) are also close to unbeatable. Druids can just escape and continue to harass you. A good Holosmith is also almost untouchable. Everything that can keep killing your illusions is hard to beat. On the other hand this makes winning even more fun.
I hope this was interesting and helpfull to some of you.