How do you kill a spider When it's on the ground in Skyward sword?

Put a bomb down near it until the bomb explodes, this should flip the spider over which will let you do a Fatal Blow on it and kill it. Also if you don't have bombs wait for it to try jumping on you, immediately do a Shield Bash (shack the Nunchuk) to knock it on it's back and do a Fatal Blow to kill it.