This is in the campaign of online Multiplayer, Lagon does not get damaged or receive skill side effects (like damned stacks) from Drain Life. I have tried using the skill basically everywhere in the arena, and all across the boss himself and his various limbs; alas, Drain Life is useless in this fight. However, Lagon is damageable by other sources (Rip Blood and Spirit Plague and the latter’s debuffs/plague in my case).
I am not sure how to relay my current state in the game, so I made a build with my skills/passives: https://www.lastepochtools.com/planner/aBZ3lmro
Many thanks for your support!