A mesmer's main job is disruption and control. A golden rule you could say is find out what is hurting your team, shut it down; and find the other teams weakness and exploit it. Example is some foe ele is laying down some hurt, you can interrupt and stop him. Or when facing a single monk, you can single handedly shut him down, and wipe the group much easier. In the end pve is still about dps over the biggest area possible. But simple reasoning like that will be worth its weight in gold if you ever decide to pvp, and I hope you do.
For skill choices, since aoe is the golden goose in pve. Esurge, ether nightmare, cry of pain, cry of frustration, shatter hex are irreplaceable.
I use this but I do not know what campaigns you have so below is an all prof bar:
[build prof=Mesmer/Necromancer][Energy Surge][Energy Burn][Ether Nightmare][Signet of Corruption][Cry of Pain][Power Drain][Power Spike][Resurrection Signet][/build]
[build prof=Mesmer/Elementalist][Energy Surge][Energy Burn][Shatter Hex][Cry of Frustration][Power Drain][Power Spike][No Skill][Resurrection Signet][/build]
The last slot is up to you, choices are glyph of lesser for no energy problems ever, inspired enchantment for enchant removal, inspired hex for more hex removal. Or whatever you wish...
For ele, searing flames really is the best elite due "its aoe without being aoe" I use this on heros, but its effectiveness is still rediculous.
[build prof=Elementalist/Mesmer][Searing Flames][Glowing Gaze][Liquid Flame][Meteor Shower][Power Drain][Glyph of Lesser Energy][Fire Attunement][Resurrection Signet][/build]
All prof build:
[build prof=Elementalist/Mesmer][Rodgort's Invocation][Searing Heat][Fireball][Incendiary Bonds][Meteor Shower][Elemental Attunement][Fire Attunement][Resurrection Signet][/build]
Prophecies fire magic is so weak, stupid NF spoiled me.
