My observations.
Merc gets half exp from Quests as u do.
it gets double exp when it gives the killing shot(than what it gets when u give the killing shot)
my 96 vl merc was getting
150 points when it kills in fairmode.
300 points if i kill any c3 or s2 creep.
so, it's obvious why low lvl players lvl up merc faster.
Important advice to low level players-BUY YOUR MERC ASAP!!
well the merc gets more xp when it hits the final shot , if u dont believe me put ur merc to attack closest enemy mode and dont attck urself in a easy map compared to ur merc and then observe ur merc xp. and then also observe the xp ur merc gets when u kill that should clear ur doubts. but yes the even if ur merc kills the creeps u do get xp.