The Mesmer, by far, has the hardest time hitting multiple enemies hard enough and fast enough to get kill credit. However, the greatsword's second, third and fourth attacks are great AoEs that have a decent cooldown time to make it an effective weapon. Utilizing this, and shatters, you can get a decent damage going to tag enemies and get kill credit.
Your traits are mainly set up to give more power to the Phantasmal Berserker you make through greatsword's fourth skill. Your other trait points are spent giving yourself strength so that when you use your other greatsword skill you can still deal enough damage on your own to get kill credit. Lastly, you do empower your Mind Wrack just enough to make it worthwhile to use at all. Getting Time Warp is just a great skill for additional, quick tagging of enemies and to be used picking up allies in mid combat.
Your utilities are all there to keep you alive. Feedback allows you to cut off ranged damage, and giving you just a bit of free damage. Null Field is your only condition cleanser so make sure you use it when you are loaded down a bit and not under enemy focus. For a stun-breaker the better choice is Decoy, over Mantra of Concentration, because typically if you are getting stunned or knocked down it means you are taking focus from enemies. Decoy allows you to break that focus and reposition yourself further from the enemy. Lastly Time Warp is there to make sure you are hitting quickly in the beginning of a fight to make up for a lack of your Phantasm.
Playing the Build
Investing so much into our Phantasm and damage means the build doesn't have a lot of survivability. The important thing then is that you are avoiding enemy focus as much as possible and that starts with not being the first to hit an enemy. If you have to attack first make sure it if is your Phantasm that is actually hitting first so it takes focus off of you.
Your Phantasm is your bread and butter when it comes to tagging enemies for kill count. It has the ability to go through enemies in its attack path, deals some solid damage on the way through and has an okay attack timer. will want to make sure that you are placing your Phantasm in clusters of enemies so that it can tag a lot of enemies. One thing to note is that if you place your Phantasm on a particularly tough enemy, like a champion, you can get long use of it and potential tags as other enemies cluster around the champion. Finally if the target of your Phantasm is about to die make sure you Mind Wrack so you can hit other enemies around that target and still get kill credit.