Mesmer is just too weak. Here is the difference: Level 10 mesmer (leveled down to 4) killing level 4 bandit 8 seconds using every ability possible. Warrior level 3 8 seconds 2 abilities 6 seconds with 3.
Possible fixes:
more clones (lots more like 15)
clones actually do damage (perhaps like 5% of the characters damage?)
clones get greater explode damage per level(please!)
clones do not shatter after the target is killed(so you can explode on other targets!)
clones can use abilities(for less damage less stun less knock-back ect.)