After several failed experiments, I will show you a way of how to download en masse from Youtube channels :
1) You must have Link Klipper addon for Google Chrome installed
2) You should have Free Youtube download (latest version) installed -- it's freeware. Google and download
Now open the said user's channel and go to videos.
Either sort from newest-oldest or oldest-newest. Put the view to grid instead of list, it will list many more videos each time you press the load more button.
Scroll down and keep using the Load More button as far as it can go. I don't know if the whole channel becomes visible if you keep doing it indefinitely, but i have sometimes run into issues with that.
Now right click, select link clipper and extract all the links. It will generate a txt file or a csv file (open this in excel) with all the links it found. Open it.
The first several links are irrelevant. What you really need are the links that go like this : https://www.youtube.com/watch?v=uDyo0ppvbI4 . Start from the first link where you see that "watch?" in the link text. All the other links following this first one will be in a similar format. Those are the videos
Shift+Select all the way down to the last "watch?" link. After the last one, you will have a few other types of links. Those are also irrelevant.
CTRL+C all the selected links to copy them to clipboard.
Now open up Free Youtube download. Go to Tools -> Options. Set the default video format to whatever you want (I'd advise not to go beyond 480p or 720p unless you have the space for it). That is the maximum size of the video that will download. If the uploader has uploaded a 240p video and no larger size is available for e.g., then that will be the size that will be downloaded. Also select the Auto Download option when you have a lot of links
Now close the options box and come to the main menu. Now paste (CTRL+V) all those links you just copied and let 'em rip.... given enough time, an entire channel will be preserved.
Just make sure you have enough space in your output folder.
Note : If you don't want mass download, just select a video and copy it's link and paste it in the Free Youtube Download window. You can then download one video at a time.