This doesn’t seem to work anymore. Has it been nerfed?
Nope the corner remains safe from the hand and the beam; you do need to be able to avoid the circular blast or to be able to tank it and recover before the next one. Ranged obviously has an easier time of it than melee. If you’re struggling against Lagon the first thing to assess is your Effective Hit Points or EHP. For him, no resists vs max resists (physical lightning and cold are the relevant ones) means double the damage taken. If you’re at max resists with at least 1000 combined HP and Ward you can tank the circular blast easily along with the claws and can even take a few ticks of the beam, you just can’t stand in it. At that point it’s just a matter of patience and not being careless, there’s no time limit.
I finally realized this (doh) - but if you get trapped on the wrong side of the beam, you can just back down the stairs so the beam doesn’t kill you.