One way is to simply be able to use a shortcut while in Windows, to quickly switch focus to the virtual machine, and then you can F12 to raise Guake or do whatever else you like.
Although VirtualBox itself has some keyboard shortcut options for focus, but I found they required too many keys, and easily conflicted with some other programs I wanted to use, so I went with using AutoHotkey portable.
install AutoHotkey portable:
The installer simply unpacks the files.
Run the portable app, it will move a copy to your Windows
My Documents\AutoHotkey.ahk where it will be easier to find
Open this in any text editor such as notepad
Scroll to the end of the file
add and save these new lines:
; raise guest machine if running or raise the manager
; allow match anywhere
if WinExist("[Running] - Oracle")
; attempt to summon guake
if WinExist("Oracle VM VirtualBox Manager")
You can update the text it is searching for inside the WinExist() conditions to better match what your version of VirtualBox has for its title bars.
To make changes effective you need to reload the script:
Go to your Windows system tray
find the "H" icon for AutoHotkey portable
right-mouse click to open the context menu
find the reload script option. If there is none then just exit AutoHotkey and run AutoHotkey again.
Be sure your VirtualBox Guest machine with Linux Mint is open, but not focused, for example example focus on notepad or something else in Windows.
On your keyboard press Win+h
This should now at least raise and focus the guest machine.
Secondly (this part untested) the script is designed to type F12 for you, see if it works for your machine.
If it doesn't, then it simply means you now manually press F12 to open Guake.