I just obtained the Monsterhunter achievement after I completed a hunting task for a medium difficulty creature -- Enfeebled Silencers. I believe you get the Monsterhunter achievement from doing 100 medium difficulty creatures as a hunting task! I didn't particularly count to 100, but if I had to estimate and I do these tasks every day I'd say 100 is reasonable. Please note, others are providing a rough estimate of 150+ medium difficulty creatures now. Bare with us while we get the exact number from someone whom actually counted. Happy hunting!
Also I did post this is the popular achievement thread we have on the official forums: https://www.tibia.com/forum/?action=thread&postid=38888225#post38888225
This question was also asked on the official forums and a CM replied leading us to believe maybe you need more than 150 tasks...I'll update when we have correct number of tasks here's the exact quote-
"I wouldn't put too much stock in statements claiming that completing 150 medium tasks will net you the achievement and instead keep going."