What Did Bro Just Have Me Fight?

Sorry about the odd screen ratio, I did this really quick on mobile. I thought he was trolling but I fought this again after and he charged at my Diluc and insta killed him so I guess he can do something. I'm going to practice this one now that I finally got Furina so expect some Diluc stuff when I figure out a comfortable rotation that utilizes things like swirling flame instead of furina's damn water all the time.