As soon as you unlock Gollum either go to a previous chapter in free play mode or keep playing until you unlock free play for the world. In free play simply set both characters to Gollum. To do this hold
to bring up the character wheel and if Gollum is not selectable in the wheel pick the bottom center selection in the wheel to bring up the full list of characters. Obviously make sure all health related extras are turned off, and then just punch the second Gollum 4 or 5 times to destroy him. Achievement should pop once the second Gollum is destroyed.