A wineprefix is some kind of a container for the virtual windows environment. It is set via a environment variable that points to the prefixes main directory. Usually, environment variables are prefixed to the launch command, e.g. `WINEPREFIX=/games/user/skylords wine ...`. The prefix is automatically created with default settings on the first start of wine if it does not exist.
You need a wineprefix before you can setup skylords. Since you also need dxkv, this is done easiest with winetreicks: `WINEPREFIX=/games/user/skylords winetricks dxvk`. Then you can copy the game into the prefix and setup your launch command.
Mangohud is a pure Linux software that is just installed globally to your system. It needs no special setup for use with wine. It is activated through the environment variable `MANGOHUD=1`.
Putting all together, you should be able to launch the game through:
`MANGOHUD=1 WINEPREFIX=/games/user/skylords/ wine C:/Battleforge/Battleforge.exe -online`