Realm first mythic +15 resto druid here, I just wanted to give some tips for bosses (for everyone) to achieve success in cos with tyrannical. During our progression we did cos+13 with those affixes 2 chests(after multiple attempts and perfect buffs) so I'm sure my strategy works with any comp, at +10.

1st boss: make sure you have someone to poison him, you don't need to click on the beacons, but the ones that are in your way to the boss, just focus the boss down and dodge stuff, you will be fine with bl/drums etc, the adds he summons will disappear once he is defeated.

2nd boss: make sure you are stacked on top of the boss, so that imps spawn very close to each other, plan out cc for each pack, and move the boss in circle. Interrupt the debuff the boss casts, 1 stack can be lethal. Healer save your cooldowns for 30-20% boss hp imp spawns, since she will gain fire bonus damage that affects imp damage too.

3rd boss: if you did everything correct and you are still in time, you will have hero/warp/drums back up for this, if not just wait for the sated rebuff to expire, you will not be able to kill the boss without it on pull. Ranged stay max range. With 130% movespeed at max range you can just walk out of the charge, moving sideways, without using anything. REMEMBER THAT THE BOSS WILL CHARGE AT THE LOCATION YOU ARE IN WHEN HE FINISHES THE CAST OF IT, NOT WHEN HE TARGETS YOU(I have seen many fail to realise this). The slicing maelstrom is unavoidable damage, so don't try to move out or anything for it, it's pointless. Like last boss, healers save dropdowns for the last 2 maelstrom, as they are the deadliest, coordinate personal cooldowns to survive the last ones. Dodge the circles on the ground that will stun you, and the moves the boss does on the ground (like first boss). Hope this helps :)