After quite a while I finally found a decent and free program which downloads all videos of a YouTube channel.
Too many people made a crap version and often even restricted so that you need to pay to unlock all functions (which suck anyway). Hell no, check this out!
DOWNLOAD
Note that this is a command line app so you have to switch to the same directory as this program using the command line and then type <programName>.exe <channelUrl> and it will do the job.
Also if you abort the program and want to resume later, no problem! It'll skip already downloaded videos. Even partly downloaded videos can be resumed. Best YouTube downloader and simple to use. 
Here's a video tutorial:
Enjoy,
Bully 
