Issue: this boss is literally impossible to kill on some characters.

According to my understanding, it requires to back-attack with a floating/knockback cc 5 times in a row in short period of time.

Succession Maegu, for example, has only one floating (Tag) on 6 seconds cooldown, which means it's physiclly impossible to fullfill the mechanics. Other examples, as far as I know, would include Ranger, Guardian (cause she's too slow to back-attack), and possibly many other classes.

I think PoU should be a place to show off your skill and understanding of the class of your choice. This boss not only design favours some classes over others, but actually discriminates some.

Even on classes that can theoretically do it (Succession Ninja here), there are other issues. You have to hit him with a floating IMMEDIATELY after he does his kick attack, and our floatings are very short range, and all on cooldowns. If you miss the CC it counts as a fail, so you need to be right on him prepared to do it the moment he does the kick.

If you kite him around, you won't be able to get to his back fast enough and it will count as a fail.

If you stay in short range to hit him after it, he will spam the kick attack so quickly that getting to his back to rotate floating skills and CC him each time before he deems you a failure (after 2 misses) is virtually impossible. 

He does this mechanic 3 times during the fight, for a total of 15 times. If you fail 4 of those, he will instantly murder you.

This mechanic is not fun.

Have to agree
I dont even bother try the mechanic and just rush damage on him, its just not worth the time.

I gave up on this boss years ago and what i do is every other day i run TET pit (the day it's gotha rensa). 

I know you can rush damage but i tried it five or six times and got close twice but never beat him that way either. I dont have patience for this shit so I just gave up.

I mean, it's weird cause in terms of difficulty he's miles ahead of any other pit boss.  The rest are all beyond easy,, but this one is brutal. 

All they need to do is have the mechanic happen just once. 15 times is mad. And i dont think originally they expected us to be able to damage rush him. 

I got to him for the first time just earlier. After failing a few times, I checked a guide (text and video) and tried a few more times. As Awa Tamer I have all the means necessary at my disposal, but.. I SUCK at fighting. I just don't seem to get the timing right and keep failing. After 20 times I had enough and quit. I think it would be nice if ANY CC worked OR if the CC-able time frame was a bit longer...t

It was a pain to get used to it with an Awakening Guardian. I basically run away from him as soon as he hides his weapons, then, as he comes running at me, I dash behind him and use S+RMB then quickly use succession dash (A/D+RMB) and run away, waiting for him to come running at me again.  I do this 4 or 5 times. When he equips his weapons again I melt him with damage before he starts his annoying loop again. 

It's not a fair boss for all classes, but  I can't expect PA to change anything.

Gotha Rensa should be removed from daily quests. Maybe Duran too. Can use them for getting to PEN but otherwise running into him every other day is frustrating to say the least. He's harder to kill than Kharoxia.

Knockbacks work too.
I do know that some classes can't do the mechanic of Duran. I'm not sure if there are some classes that cant do Gotha Rensa mechanic. With how slow guardian is it might actually not be possible.
But Tamer, Ninja or Ranger is just skill issue. Sorry to say it like that, sorry that it comes of as rude, but it is what it is. I've been doing Pit of the Undying since long before they made it easier when you were actually forced to do the mechanics because the bosses were too tanky to just burst them. I've done Pit of the Undying on a whole bunch of classes including awakening and succession Kuno, awakening Tamer, succ Sorc, awakening Mystic, Shai, awakening Drakania.

A few tips to help though: Pit of the Undying is 0% AP cap at 600something AP, which means none of the AP above 600something matters. Just equip as much human damage and special attack damage as possible and you will deal a lot more damage. I have a full human damage crystal setup and dehkia artifacts with human damage. On top of that you can use an Elixir of Indignation cause it only costs 1 million silver. human damage add-on and you're already at 150 human damage.
Gotha Rensa doesn't get a DP buff if you don't float him in his back during the mechanic which means you have a lot more time to dps him. Every class should be able to kill Gotha Rensa before he teleports to the middle with DP buff.