First time I played this game it unlocked while my Sister-in-law was playing. basically each time you hit an animal or extra Item (bee hives and horns) your combo will increase. once you hit 3 things in a row your combo will show up on the screen along with the extra points you earned for the last shot. get this number to 50 and the achievement will pop up right away.
Tips and tricks-
you don't have to kill the target to get credit for your combo so you want to aim for the body (This is also how you get the higher scoring "Heart-shots") *good places to up your combo are the Bear battles. each bear will run at you and then stand up. fire a few shots into his body you can get 3-6 shots on his body before he drops. I know the first level has a couple bears so you can get 15-20 of your shots on them no problem.
The groups of ducks are good places to start/increase your combos quickly. (make sure you shoot them with the shot gun as a rifle shot will kill the bird but give you negative points and kill your combo) The ducks are good because you can kill multiple ducks with one shotgun blast and each one adds to the combo so if you get 7 in one shot not only will you get
but you will add 7 to your current combo total so its easy to get your combo up quickly with a big group of ducks but again be careful not to miss or over shoot.
be careful that you do not over shoot. once the score pops out of the animal they are dead. If you shoot the dead body your combo will be lost and will start over.
There are a few ways to go about this. as you play you will get better over time and may stumble upon it like my sis-in-law did. but just playing like normal if your really having trouble then don't worry about your score for 1 run. Start up any level and take it one shot at a time. aim for the body (multiple hits on the same animal add to your combo)
I will try to confirm that you can get this combo on the survival levels because the 3rd one (with the Bear on the picture) has a ton of bears and as long as your careful about not over shooting them (shoot after they are dead) I was able to rack up 100+ combo in under 2 minuets
If you have any questions or comments on this solution just let me know- good luck and good hunting!