It's all about getting the timing down down on his special dexes while rooted. Personally, I found the sp1 to be the much easier one. In phase 1 & 2, I tried not to move while dexing as if I was rooted. Took me a few tries. I also found interrupting of heavy to be easier as well. Dex 1st hit & retaliate, rather than trying to dex both hits. Definitely struggled more with heavy than specials. Good luck
Learn the dex timing mainly. If that's giving you trouble you can use fanta and if you ever get clipped his glancing saves you. I used him and took down phase 3 every time no problem.
Other things that could help is baiting the sp2 instead of 1
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Other things that could help is baiting the sp2 instead of 1