Welcome to Smogon! Take a moment to read the Introduction to Smogon for a run-down on everything Smogon, and make sure you take some time to read the global rules.
The definition of a counter is a Pokemon that can switch into any move that the pokemon its countering and OHKO back. Mega Manectric is a counter to Scizor. I didn't want to make a one-liner, so thought I'd just tell you directly :P