I haven't had any difficulty pulling off either the spin attack or the finishing blow. The second is just bring both the remote and nunchuk down simultaneously; works every time.
The one portion of the sword play that doesn't work reliably, for me, is the stab. I tend to get downward swings when I try it. It's a little frustrating when trying to take out enemies that require it, likeBeamos in the third dungeon.
Make sure you don't move the nunchuk, I always keep the nunchuk to my chest and do a stabbing motion with the remote, it never fails for me.