128652
There are two relevant damage mechanics in this fight: the 274991 debuff and the 140447's 269266, and the way you handle how they overlap during the fight is the key to killing the boss or wiping.
First, for 274991 it will apply to two random players every 20 seconds, dispelling this sends any other player near the affected one go flying away so your group should be lightly spread allowing you to instantly dispell someone without fear of knocking someone into the water. The debuff does heavy damage on application and ticks for a decent amount so it's very important for you to instantly dispell one and very quickly triage healing the other affected person to reduce incoming damage.
The 269266 from the 140447s will deal heavy damage to everyone, you will constantly have to deal with this damage for as long as the 140447 is alive.
The hardest part of the fight is when these two mechanics happen at the exact same time. You are guaranteed to have this overlap on the second set of dispells 25 seconds into the fight. However after that it will heavily vary depending on what you do with the 140447s. The boss will keep casting 274991 every 20 seconds of fight but the 140447 will stop casting 269266s when you kill one and start again when you shoot the cannon at the boss
Another consideration is that each 140447 on each different platform casts 269266 at different rates. For the start of the fight you will get one 269266 approximately every ~18 seconds, while on the second platform they will come every ~14 seconds and on the last one every ~10 seconds. This means that overlaps between 269266 and 274991 will be completely different depending on how fast you kill the 140447s and/or how much time you take to shoot the cannon after that
As you can't know before-hand when this is gonna happen, i heavily recommend you go into your boss mods settings and turn up the alerts for these two mechanics. Knowing when they will happen at the same time and using cooldowns appropriately to handle that is the only important part of the fight and if you can do that the boss doesn't do anything else.