Point the 1st: I'm fairly sure the map mods of "faster casting bosses" or some such was effecting the beam, it was moving faster than I see in various yuotube videos. I am wearing move speed boots, it wasn't fast enough when I couldn't run in a straight line due to void zones with the map mods on the map.are... are you playing a build with zero movespeed from tree or boots?
Why would you EVER do that to yourself unless you're using whirling blades?
I think we found the problem. Get yourself some boots with 20-30% runspeed on them because playing the game without some basic runspeed is genuinely awful. This isn't "you need a special build", this is "why would you ever want to be slow?". Endgame is balanced around certain levels of damage, survivability, and yes, mobility. How you reach those threshholds is up to you.
Also, the pack of mobs is a blue pack, they should be dead within a few seconds maximum.
Point the 2nd: After search for "malformation piety" on youtube, I've watched 4 videos worth of attempts. Basically, the top 4 results for that search. 0/4 videos show the player actually avoid the beam 100% of the time. None of them. These are streamers who do this stuff for money and they can't avoid the beam, it seems it's not as easy as you are trying to imply.
Point the 3rd: I can't stress this enough: a hard fight is fine if there is some reward involved to make it worthwhile. Oops, they forgot that part. Any other T9 map boss I can kill in my sleep, without move gear or changing out my gems. Even if I could kill piety without dying a single time, why would I ever bother when I could instead do a different T9 map and roll some "dangerous" map mods, enjoy greater IIQ IIR, and still have an easier time with the whole thing?