pet exp is based on how many hits, so what you want is a ton of defence and as little attack as possible
so basically you want to take your weapon off and hit a monster with your hand
the best thing you can do is have it so you do less damage than the monsters health regain (so there health never get to 0) and so the monster cannot hurt you enough to kill you where if you walk away you can come back and you will have NEARLY full health
so you need a high defence and low attack
one way to do this is to make a low level knight, low as possible and get as much defence as possible, then attack a sasquatch, pull it off somwhere remote so no noob kills it, and train
if your level 25-27 i recommend fighting a Treant or Butcher
if ur inbetween i suggest drakes(not elders)
if you can do this right (which i have not yet, treants last 13 mins but then they die and i need to get a new one because my attack is too high) but anyways if you do it right you can train a bet, leave your pc on while u sleep or whatever, then sell the high level pets you got for tons of money