So my friends want to play a Witcher like campaign. When i imagine Witcher i think of Geralt on the trail of a huge beast, stalking and studying its behaviors and habits. After studying the creature you spring a trap for the beast and slay it using the tools you have and the environment.
I am also thinking to make it a contract based missions. This would allow them to choose what monster they would like to kill as well as set bonus objectives where i can give them incentive to make things a bit more difficut (like kill the hulking beast without overly damaging the hide, so no fire or acid attacks). 
Dose anyone have any advice on how i might go about this or any monsters that you think would be fun to hunt? 

I think this is a seed to a possible adventure, and with a lot of care could grow into a campaign - but I think hunting just on its own doesn't make for a complex multi-level story ( a series of adventures which are, themselves, wrapped in a larger encompassing story ). You'd have to add extra elements to shape it into one.

Overall, I think the contract hunter framework is a good one. It gives you an ability to hand out small seemingly unconnected contracts, while keeping the Party and the story together, and would allow you to start weaving the individual missions together into a larger narrative

For an individual hunts, it might be tricky to keep the adventure pattern interesting over many such missions, so you'd have to evolve the adventure pattern mission to mission.

For the first hunt, something along the lines a Skill Challenge, capped with an interesting resolution when the Party catches up their prey ( most likely a combat, but I never bet on predicting Player actions, maybe they'll concoct an elaborate capture trap ). This is essentially a really slow paced chase so I'd point you to How to Build F$&%ing Awesome Encounters! by The AngryDM, whose article contains a chase based skill challenge.

I'd also put in a number of environment based Encounters along the way: don't forget that if they're trekking through hostile wilderness, that the environment it'self can pose challenges for them to overcome. There's also nothing to prevent them from running into conflict with other creatures in the area, or even other sentient beings.

With a little creativity, you can spin up a relatively simple, yet enjoyable, first hunt.

Beyond that - I believe that you'd have to be seeding the hunt missions with larger narrative clues to keep the narrative rolling. What is connecting all these hunts? Is there a particular contractor which has some plan, of which these hunts are part, and if so what might that be? Does the Party develop a rivalry with one of the other hunter teams, and some of the hunts become competitive races? Does the Party do something hunting their prey which earns them the enmity of someone powerful? Does the Party "coincidentally" keep running across clues and ruins of a particular type, and their client seems more interested in hearing the story of their adventures ( especially in those ruins ) than in the spoils of the hunt? Is there a faction, or individual, in the leadership of the hunt organization which is testing the moral limits of the Party, sending them again more and more intelligent "animals", testing to see if the Party is willing to be used as assassins? Some, or all of the above?

Only you - and the Player actions - can decide what the larger conflict connecting all the hunt missions, and what kind of Campaign you want to create here.

Have fun :)

Use the Witcher as a basis. I am let at night so keeping it brief. The main focus was not the hunts, the overarching story to save his adopted daughter was. Unless going with the above mentioning all are tied together maybe just have preplanned hunts they can go on for extra flavor, rewards, and the like. Do update if you go through with it. My friends all love the Witcher and an idea of how to build hunts for them is rather cool.