I found that there are three variations of killing him that I enjoy.. they can be intermixed for whatever the situation allows.
First, you definitely want to get rid of his shield.. this can be done in one of two ways:
1) You can slice horizontally and vertically on his wooden shield, and eventually after you get it to about a quarter of its size, he will toss it away. This makes his soft, squishy belly vulnerable for attack.
2) You can chuck a bomb at him, and he will back away while holding the shield as up protection.. The bomb should blow his shield up if tossed correctly.
From here, you have a few options, but here are a couple of my favorites:
1) You can slash his belly or rear section a ton of times, and he should go down. Watch out for his attacks while you are doing this, as he will be trying to approach and hit you in the process. If he starts rearing back to attack you with the spear, do a shield bash right before/when he starts his attack and it will knock him off kilter. This should allow you to slash away some more at his soft, squishy belly. I enjoy this because it has a more personal feel to it, it's more exciting, and it reminds me of jello. It should be noted, when you are slashing his butt/belly, it may look like he is not getting hurt.. He is though, it just takes a lot of hits.
2) You can blow him up with a bomb. He backs away very slowly, so if you roll it/throw one or two just right, it will kill him. Also enjoyable, but more from a popcorn theatre point of view. I opt for the first method though, as I tend to be pretty stingy with my bombs.