Mythic changes
Unstable Portal starts casting after 30 instead of 45. This will create small burn phases on every platform and prevent delaying entering the portals because of tank swaps. Probably best way would be holding dps at 92%/62%/32% mark until tanks swap.
For the first platform (fire) three people with high mobility need to go to green platform first just before fire opens and get the debuff from pools there and explode it on top of the fire miniboss. Miniboss has a shield with three stacks of -30% damage reduction so each green debuff dragged here will increase damage significantly. Green debuff will explode in 10y radius and all raid except tank needs to be away from it when damage reduction debuff is stripped off. Stagger these with few seconds between to avoid debuffed people exploding each others.
On green platform the miniboss leeches health off from players. To prevent the healing few players must run to the shadow platform and collect the purple orbs from there to get a healing debuff. The debuff lasts for 30 seconds and when it expires it reduces healing of the miniboss by 100% for x seconds and all players who are too close. Probably need two people to get these orbs, staggered so that one arrives there with debuff not long after the portal opens.