Dealing with the first boss efficiently is vitally important - generally you can and should save all your cooldowns for the first burn phase, but if the keystone level is high enough that you don’t kill the boss during that phase (which will be true for most groups at most 15+ key levels on Tyrannical weeks), you should then start using cooldowns as they become available to escape the non-burn phases or else you’ll not be able to survive until the next burn.