There are two ways to kill it during its third phase. You can stick to its butt and attack its tail, or you can just keep your distance and attack its arms. I don't like being very close to it because it makes it harder to avoid attacks.
Here's a video I took of my fight against it. You can see that it takes me 4 minutes to get it in its third phase and then 7 minutes to finish it off - I was pretty stressed and kept on missing my chances but as you can see it's fairly simple to avoid its attacks when you stay at a distance. I'm sure I could have killed it in < 2 minutes after getting it into its third phase if I didn't miss that much.
The attack you need to learn how to avoid the most is the one where it slams each arm down twice in a row during the third phase. If you spam the dodge button you'll get crushed. You need to dodge once, wait for a second or so and then spam the dodge button.