This is how medusa should actually be played as a 25 min farmer that can be borderline unkillable when given the items. A common item people tend to go for when playing her is manta style. That may be the worst item to use. For starters her split shot causes her to give an output of 80% with manta, ur illusions deal 30% of the 80% reduced resulting in 24% output per illusion. that give a total of 129%(24% + 24% +80 %) of a damage output. In my opinion thats nothing.
Your play style should be elusive not riki or slark elusive but natures prophet and tinker elusive. You should be warding the jungles every chance you get, with the farm the amount should be nothing. When you get boots of travel not only will you have reliable speed you will be able to switch between empty lanes which allow you to farm. And if youre in a senario that you get ganked or ambushed by a team always remember.. wait a good 3 secs to make sure every hero ganking you is there (so no straglers like sven or wraith king can pop a stun) so you can stone gaze and immediatly use boots of travel to teloport to the next lane.
Once you get linkens you should be helping in team fights as much as possible, mana shield on split shot on and snakes ready. When in a team fight ALWAYS use the ult half way into a fight and not while chasing. For some sort of reason if you initiate with stone gaze everyone stops fighting during stone gaze like its naga's siren ult, so keep in mind and walk in the middle of thier team then use the ult. It is suggested you do it during a fight to allow allies to escape or to finish the job