Oops, sorry I forgot to place a data between quotation. The original post is edited!
Just before, I have to add that this emu will need a script to quit, so the emu name needs to be adapted. From 'Cemu' to 'Cemu [Script]'
The code is now:
<System type="Console">
<Name ss="18" em="Nintendo_Wii_U" tgdb="" gb="">Nintendo Wii U</Name>
<Emu selected="Cemu [Script]">
<Cmd hidden="false" name="Cemu [Script]" value="-f -g "%path\%file%ext"" extensions=".rpx,.wud,.wux"/>
</Emu>
</System>
As for the script, take the first one in 'Script' folder (1964, 1964 UltraFast [Script].au3), copy it then rename it to: Cemu [Script]
One last thing, an icon for the system! In Data > BaseSystems create a folder called 'Nintendo Wii U' and drop the joined icon.png file into it!
Tested working!