A good spot for this is S12_TALONPIKE on \\NETWORK_2A. Use the Breach point at the top of the map to enter the server, and you'll see two regular guys walk right past an explosive cube. You cannot shoot it directly, as servers don't let you automatically start -- there's a countdown process before you can move or shoot. Stealth to the cube, place it next to where they come to rest, and blow it up. It doesn't matter if you die (speaking from personal experience) just so long as they die.
- If your enemies are too tough, consider using nukes to reduce their health/armor, or simply find a different area. I know there are more spots this could easily be done, but I don't remember them off the top of my head.
- You can increase how much damage an Unstable (Red) cube does when shot using weapon abilities. I got one in a pack for my Combat Rifle. If you happen across one, that could help.