im a subt rogue so this is what i usually do if i want someone stunned for a while

cloak
premed
shadowstep
cheap shot
backstab
kidney shot (should last 6 seconds)
backstab
gouge (as soon as kidney shot ends)
backstab (right before gouge runs out or you get full energy)
hemo
sinister strike
kidney shot again (again 6 seconds)
shadowstep backstab / shadowstep ambush (if you can get cloaked)

if the fight isn't over by now i either blowing your cooldowns and going all out or buying some new weapons.