After maybe 15 seconds into the first phase of the Lagon fight, he just stopped attacking. I do not believe he ever used the lightning ‘sweep’ attack but he did use the big AoE in front of himself once, and right then is where he just stopped fighting back. There were still the small lightning AoE’s on the ground, but he otherwise just sat there and didn’t use any attacks. After the 2nd phase, the third phase was the same, he just sat there and let me beat on him.
If you want me details I will help if I can, but I didn’t record the fight. My weapons of choice were Rip Blood, Aura of Decay, Bone Curse, Infernal Shade (transmuted into Blood and Blaze Shade), and possibly Transplant but I don’t know for sure if I ever used it. I just made a guide about this build if you’d like to read it (shameless plug!)