1. Download Xcode from https://developer.apple.com/download/more/ (this requeires to Login in with an Apple Developer Account)
At the moment id don´t know how to authenticate so i have no clue to download the xip via curl/wget.
In my case i downloaded the file and copied it via scp to my mac.
eg. for Xcode 9.2 https://developer.apple.com/services-account/download?path=/Developer_Tools/Xcode_9.2/Xcode_9.2.xip
pkgutil --verbose --check-signature path/to/xip
eg. pkgutil --verbose --check-signature Xcode_9.2.xip
xar -xf path/to/xip
eg. xar -xf Xcode_9.2.xip
curl -O https://gist.githubusercontent.com/pudquick/ff412bcb29c9c1fa4b8d/raw/24b25538ea8df8d0634a2a6189aa581ccc6a5b4b/parse_pbzx2.py
python parse_pbzx2.py Content
Locate all Concent.partXX.cpio.xz file with more than 0Bytes and decompress them
xz -d Content.partXX.cpio.xz
for i in *.cpio; do cat $i >> Content.cpio; done
sudo cpio -idm < Content.cpio
sudo mv ./Xcode.app /Applications/
THIS was the only working solution for me.
The system volume of my mac mini has less than 30gb free space, so no app store install would work.
On my second HDD I've got plenty of free space, but the Archive Utility always bloads my system volume until Mac Os crashed.