Sigh
How do you catch a LIVE overspawn... not a megaspawn.
the easiest and most cost effective way to catch a MEGASPAWN is to hide in the cloud of a smoke grenade (well 2 spaces at the edge of one that has blown) you dont need any powerfull weapons to take one on, ive killed 2 with agents armed only with sniper rifles and machine guns)
Ps. An overspawn is the creature that destroys the city and is not found in any agent based battle.