Very challenging boss fight.
We have access to some new things though to help us out. Make sure your harvesting gold coin fruit from the tree at Hotel Krat. You can use these at the vendor to get better wishstones. I like the ones that buff your specter with elemental damage. I used the Lighting Wishstone for this boss.
We also now have access to the vendor in Malum District who sells throwing objects. I went to him and bought 5 power cells, 3 saws, 3 cluster grenades and equipped them to my toolbet. You could buy even more if you wanted.
This is sort of a cheese method that worked for me.
Phase 1 - Cast wishstone and weapon grindstone/abrasive and try to blow the boss quickly. Use pulse cells to stay alive. Hopefully the boss stays on your specter and you can phase the boss rather quickly.
Phase 2 - (If you got to phase 2 and you still have your specter and some pulse cells I recommend trying the cheese method, if not wipe and try again so you don't have to go buy throwing objects every time.)
Once phase 2 starts I waited for the boss to attack the specter and started throwing all my thrown objects at him. I also used the falcon eyes legion arm to damage him at a distance as well. I was able to get him to about a 1/4 hp this way. After that just be patient use fable arts if you have it and try to guard his fast attacks.