I play a mesmer and have done some explorables (mainly Sorrow’s Embrace). My fastest explorable run was with a group of 2 mesmers, an elementalist, a warrior and a guardian (Sorrow’s Embrace about 32min). I can agree that mesmer has kind of a “bad damage” for some fights because you don’t have the time to setup phantasms. For boss fights when you have 3 phantasms up your dps is good.
I personally run with sword/focus + staff, a lot of CDR traits, Blink, Nullfield and Phantasmal Disenchanter/Illusion of Life, which is a great support skill, someone goes down near the boss -> you can revive them instantly and get them of the boss. Although this needs coordination because the one who has been revived can’t see a 15s timer/buff called Illusion of Life. Time Warp is also great, as addressed here before, because it buffs your teams overall DPS by about 50% for 10s every 3.5 minutes. Oh and also the Temporal Curtain and Warden reflects projectiles if traited (OP against ranged mobs).