Impale is is great ability, but you need to make sure you land it right. So what you can do is actually hold (on the GameCube controller) A and B at the same time while tilting the stick to the side. So it's basically your side special but you're pressing A and B at once. That'll automatically impale terrain if you're on the ground. Let me know if this works!
With your side special, if they're close behind you, you can kick out in the opposite direction and kick them a little with it too. His counter isn't as good as it used to be but still a good option. His nair is excellent still. Let me know what you think. I'm by no means a pro but I've spent some time with Corrin so I'm used to all his stuff.