UE condition #2: Shaper's beam, any tips?

Slowly but steadily, I finished all boss conditionals, but I continually fail this one, 8 times already.
Most boss conditions was pretty easy, some required some adaptation, some just required more tries.
Like I just finished the Maven ones lastly right now in 4 attempts, 1 by 1, focusing 1 at a time.

But somehow I cannot do this one; fun fact, I was the closest for this on my Uber Uber Elder run.
In Shaper fight, avoiding the beam was so easy, I did that parallel with the no detonation on first try.

And to farm up an UE set with own maps takes ages for me, so would like to put and end at it.
Not to mention, that I lost already tons of currency on them with pointless drops..

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Defeat The Elder in The Shaper's Realm while fulfilling each of the following conditions.

Without taking damage from The Shaper's Beam

I tried to be on the Shaper all the time in melee range, still got hit, once I swear clearly behind him.
I tried to be a screen away but having my eye on him when beam comes, still got hit eventually.

The closest I was unbeamed till the second elder portal phase then got slowed into it in a split second.

So, any best practice / reliable tip for this specific one remaining condition?
The fight takes very long, and Elder's slowing messing it up.

Best way probably is running in a circle non-stop around Shaper or in a circle non-stop around the arena, trying to keep him in your screen all the time, not at melee range, but a bit farther away. The hitbox of the beam is bigger than it shows and also it starts a bit before the animation, leading eventually to a few failed tries.

Since you are looking only for the achievement, go with 5 quicksilver flasks and always use one when you are in a bad place or slowed by Elder. Also, you are already level 100, so, just letting yourself getting killed by any other mechanics in order to not get hit by the beam can be a viable option as well (or to reset the flasks if you ran out).

Uh, probably decoy totem can help as well, no clue by how much but, you could try on other fights and see it if benefits you somehow.

Thanks, the 4-5 quicksilver flasks option sounds good indeed for the slows!
About the hitbox, I had that feeling, especially when I got hit seemingly behind him.

If I remember correctly, he's MORE likely to beam if you are in melee range. Can't really give more tips but I will say a conditional I had a few leagues ago really helped (defeat the shaper without getting hit period) because I was really bad at the Shaper fight and that challenge forced me to practice a crap ton.