Yeah okay with you 10s of thousands of degress (btw the surface of the sun is 5000 degrees Kelvin).
How would you aim it? Also plasma would burn too quickly, flamethrower chems are designed to burn slowly enough so there is still something to hit the guy with. You are not just hitting the guy with flames you know, there is a good amount of burning chemical goo that sticks to the target.
Plasma temperatures are simply an output of controllable heat induction (typically via lasers relative to our technology, not sure what the aliens use), thus they can be calibrated to a desired level. At its highest charged states, it indeed features temperatures at tens of thousands of degrees centigrade, while 'cold' plasmas are thousands of degrees centigrade.
Second, I would assume the plasma is cohered for a short distance via a temporary electromagnetic field (it also occurs to me that 'marking' a target in an emulation of lightning phenomena, or an electrolaser/aerial conduction methodology could also work), the whole process of its projection, plasma generation and discharge being activated by a trigger. Aim the discharge port at your target, hold down the trigger, and you're done. Further, precisely because plasma is so hot, it doesn't require continual burning to annihilate its target. A short burst should be more than enough to destroy almost anything; the initial exposure is incredibly deadly.