Rightio
Putricide is as you say a very simple boss.
We have Putricide tanked near where the green slimes spawn, with the tank facing him to the wall at the North (green slime) side of the room. The idea is that the tank kites him to the wall when he dumps his wtfpwnmakesyoumiss orange flasks. The flasks get dumped out of harms way and then putricide is brought back. Melee dps him with backs to the centre of the room (to give an idea of positioning). He seems to stand near the outside of the big circle.
When a green slime spawns, all melee run to it ready to pounce as soon as it spawns. Don't stand under the slime fountain because it hurts. Pewpewpew, at most you should get 1 explosion if it targets a melee, otherwise you should get none
Gas cloud is simple, kite pewpewpew. All this time Putricide stays in the same place, northish of the room. Maybe a small bit of movement for slime pools / flasks but that's all. less movement = better melee dps / less ranged repositiining = less chaos. Melee move in as soon as a target is chosen to pewpew it down faster.
Ranged (and melee when dpsing a slime) should dodge the maleable goo, if they don't then they fail and probably cause a wipe. Also the goo gives a nasty slow debuff that hurts caster DPS/healing.
That should be P1 + P2, P3 you should aim to have no slime puddles, or as few as pos. You should also aim to have no slime, a slime up is probably a wipe due to not enough DPS on boss/dead dps/tank. If a tank dies early on in this phase and has not had more than 1 debuff stack on him it is not a wipe, just get them up ASAP and be ready to heal like a mofo. We had our second tank die twice on our kill in the start of P3 - once because the healers diddn't realise how fast he would die again
Essential bits of P3 -> melee stand at max melee range to avoid orange miss pots. Ranged ALWAYS stand at ranged, if too many are in melee range then goo will wipe you 100%. It is more important for ranged to stay at ranged than to stay alive to a degree in those last few seconds, becasue goo in melee == tank death == boss heals == wipe
Transitions are a bit point here, aim to get Putricide to 82%. This should be doable either when the first gas spore spawns, or shortly after. Once the gas spore is about to die/dead then dps him to P2 ASAP before the green spawns.
P2-P3 is the more crucial one, you can recover from a bad P1-P2 transition easily but P2-P3 is effectively the start of your soft enrage countdown as I'm sure you know. It is crucial 100% that you have NO slime, gas or greenie, at all spawned when he tear gasses. It is not the end of the world if you have a slime with say 20-40% health when he gasses because dots will probably kill it but still
Do all that, then heroism 20 or so seconds into P3 - whenever you are comfortable - and you should have a kill
Losing a tank in P3 if they have many stacks is a wipe so that is healer priority
tanks + ofc themselves to heal the tanks
Hope that is of help