Just because a fight is passable doesnt mean it is not bugged. My group has had this all on farm for many weeks but are also aware of all the bugs.
I honestly dont know what entering the challenge has to do with rising slash. Maybe you are thinking of spinning slash? Rising slash is one of his knock up abilities put on tanks.
Regardless the threat on this fight is really messed up but mostly it is because your tanks get an armor buff and threat debuff from the crystal. The best way to combat this is to taunt fluff constantly. Also we always tank swap after force execution cause it hits hard and is usualy followed by a hard rising slash.
Again... This fight is doable but very bugged. It is incredibly frustrating to have tyrans move to a different platform even if the tank who has agro has not moved. Jerba is explaining things that have nothing to do with the problems that really occur on this fight with position. My current theory is that it has something to do with latency. Anyway, i found it best to keep tyrans on one spot and have the tanks swap on that spot then run simplification away.
I have read different theories on when the Rising Slash happens, and I currently believe that it always happens when all three challenges are completed. If your tanks and healers are going in first, they have a few seconds to position the boss correctly and be ready to heal the tanks as soon as the DPS finish their challenge.
The aggro loss always happens as soon as that large AOE is over.
Yeah, not sure why Tyrans is sometimes jumping to a different platform. I agree that it is best to have both tanks stand on the same platform; that way you don't have too much trouble if the boss is jumping to a DPS.
Feel free to file a bug report about these issues, but please don't call them bugs, just say that you have experienced certain things you cannot explain with the mechanics, and the devs should decide whether or not this is a bug.