If you can't see the terminal, then that's pretty bad so idk -> if everything goes unresponsive I just slap my monitor in impotent fury and reboot
If you can see the terminal but not the window, idk if xkill would work. Then you'd need to find the process id and kill it with pkill.
Like say you're playing age of empires 2: pgrep aoe (should return all running processes called aoe with their pids > process: aoe2 pid: 69420 ...or something like that) then: pkill 69420 > ded