Well I planned to never ever do this but hey.. Not much else to do atm.

Could someone help me out with a list of the achivements needed and the less easy ones with some sort of small guide or whatever. Ill give away 96,37 fg for the help.

Make it count

http://steamcommunity.com/stats/DungeonDefenders/achievements/

Least completed one storing all pet types in the item box. I would say just buy random pets off the tavern vendor. Youtube the insane challenges although most are easy to figure out on your own it just takes execution. One of the actual hardest ones may be just completing every map on medium with 4 players but if you can find 3 more who just want to burn through campaign for the achieve it would probably go quick.

Mm, I did the 4 player one naturally. Only missed a few I had to go back and re-do. Make/join pub games on insane and do gunslinger/master strategist at same time.
Insane Assault was probably hardest for me. Insane Zippy Terror, Treasure Hunt, and Wizardry you'll probably need others to help as well.
Most of them are just easy, but take awhile (reach 20th level of survival, gunslinger/master strategist)

Pretty straightforward otherwise

What does gunslinger even mean? I cant see any description on what to do... :S

Is there anywhere I can see how many and what maps I did with 4 players? Cause I aint got that one yet.

Goto your crystal and view your statistics. It shows you if you've done teamwork, gunslinger, master strategist, flawless victory, skin of your teeth and such on whichever map.