I was fighting Lagon with my Smite caster Paladin build & was able to cheese the first phase by running up & round the platform that you fight Lagon on then down into the main area. As I was running down into the main area the tentacles appeared & blocked me from getting back up, however I was able to damage Lagon without being hit by standing up close to the tentacle & hit in with Smite. All of Lagon’s attacks missed as I was either on a lower level (his laser) or out of range (his claw attack), his big circular AoE was able to hit me but could be run out of easily.
Once I’d pushed him to the second phase with the adds, the fight proceeded as normal (kill adds, get teleported up to Lagon’s platform).
You might want to add a check at the beginning of the fight such that if the player isn’t on the platform then you teleport them onto the platform.