The equation used for determining damage with this weapon would be nice, then we could figure out if your situation is an isolated case (i.e a bug or deficiency on your part, e.g not enough skill/stat in something) or indeed if the weapon needs a boost.
Way I see it, shooting a bolt of plasma (whose temperature is in the 10^4 K range) should completely incinerate an enemy; its equivalent to being struck directly by lightning. And since the weapon is called the “Fusion” Blaster Arm, I would assume even higher temperatures (10 000 times as large) and much higher energy costs associated with firing such a weapon.
Though realistically, with our current tech, such a thing would be impossible (the miniaturization especially, is implausible), I’m still interested in seeing more weapons of this nature (beam weapons especially); they should be hard-to-find things and extremely powerful.