None is preset that I am aware of. But you can set one in Settings > Hardware > Keyboard Shortcuts. Click add, choose the (previously unused) combination you want to assign, and browse for the application mate-system-monitor.
Or use a terminal window and the "top" program. Or command line with the "ps" and "kill" command and their options. You will find that most of the power available in all Unix/linux is at the command line--GUI admin applications are generally just interfaces to other actual command-line programs.
You definitely need a different approach: Alt-SysRq-B doesn't unmount filesystem nor sync files. It is asking for data loss.