You can make use of the left ramp and hit with ranged attack/spell. Or simply melee tentacle and watch for moon blast on the floor (white circle) or eye beam (eye turns red and then he does it), and go down the ramp then back up to avoid getting hit by those two mechanics.
hahahahahaha, made my day
First time I fought Lagon I uninstalled the game after several tries.
Nah sorry it’s as easy as that for real. I fight Lagon on auto pilot because it’s very easy because you need to dodge ONE skill and the waves in phase 2. Nothing else because his stuns happen in at a time where they have 0 impact on you and only reduce your damage output.
his comment made my day as in what he said was funny. It was not meant to be sarcastic.
So I had to fight him in the monolith system where he is even harder than in the campaign and at first, I felt like you do.
-I had to watch a few vids.
-Also found the fight was WAY easier if I truly relaxed my mind and treated the encounter like Tetris or whack-a-mole.
-He has a couple one shot mechanics. Stay moving back and forth to the tendrils and STAY relaxed.
-Some boss fights in this game are a bit too sweaty for my taste but I would rather they not dumb them all down and take that challenge away from the players who WANT it this way.
-I am also well beyond 20 and my reflexes are not as quick as they used to be but I promise, you can do it. RELAX, keep track of where his eye is facing, don’t stay still, weave back and forth between the tendril things.
Yeah but I don’t meant to make a joke 
. I got what you did but I want to make sure for other people that I ment what I said so they look for the problem between the chair and the screen.