Guild Wars 2

The only real reason people get hit on walls by iBerserkers is because they’re within the Mesmer’s line of sight when they decidedly back far away enough from it to formally do so, or attempt to do so anyway. It doesn’t exactly help the other party when they’re using a siege weapon mounted on the wall, while standing significantly close to the edge, and hence be within the Mesmer’s LoS; so really, shouldn’t it be the player’s own carelessness for standing in the Mesmer’s LoS in the first place to blame?

It’s worth a shot…

Decoy doesn’t seem to work for me at times; I activate the skill, spawn a clone, yet do not become invisible throughout the entire process.

Being an intermediate Mesmer-user myself, the way that I see it, is that clones generally have more health than phantasms, the former being augmented for pseudo-tanking and/or (mainly) shattering, and the latter built for DPS while lacking a great deal of heath. You could also try adding Signet of Illusions to a utility slot and seeing how that fares for you (it does for me), although most of the time you’ll find yourself summoning out new phantasms as soon as viably possible, in both PvE and PvP. if you have any intention(s) of summoning phantasms as as quickly as possible, I suggest you allocate traits into Phantasmal Haste (X – 20 Illusions) and whichever trait(s) accommodates for your weapon(s) of choice, which also lowers the cooldown of their respective phantasm; although really, it all comes down to when and where you decide to summon them (phantasms) out.