This was overall an enjoyable encounter not because of difficulty but, because you have to be fairly certain of your DPS.
Starting off the fight we will be met with the Darkness Falls phase which means Lu'lin will be empowered and the sister consort will be unavailable for extended periods of time.
During the Dark phase Lu'lin will channel an ability called Cosmic Barrage. This ability will summon a barrage of stars which at the end of Lu'lin's channel will crash down upon a player dealing 23,000 to 24,000 Arcane damage (Feint-able) for each star and cause a knock back.
On Heroic Lu'lin has access to a new ability called Lurker's in the Night. This ability summons forth Lurker's that will continuously cast Darkness at you dealing 3276 to 3445 Shadow damage. The damage is moderate but builds up incredibly fast. The way this is supposed to be handled is via Suen's Tears of the Sun ability which will expose nearby Lurkers to daylight causing them to no longer cast their Shadow Barrage and instead engage in melee range of the player. Unfortunately the build up is too fast for this to be of any use so we must deal with it.
The rest of Lu'lin's abilities are not too much of note however, it is imperative that whenever Suen appears for Tears of the Sun that you switch and DPS her for the duration. This will play a key role for the rest of the fight.
Once you have killed Lu'lin the Darkness Falls phase will end and Suen will engage with the Blazing Daylight phase. Even if you do not kill Lu'lin the boss will eventually hard phase into a Daylight phase and you will have to deal with Lu'lin's Ice Comet ability.
Suen at first is not very threatening however she will periodically gain Blazing Radiance. Blazing Radiance pulses every 2 seconds dealing 4095 to 4306 Fire damage and additionally increases all fire damage dealt by Suen by 5% for every pulse. Periodically Suen will apply a debuff called Fan of Flames. This ability causes the target to take 25% increased Fire damage for 30 seconds. This effect stacks. Fan of Flames coupled with her ever increasing Blazing Radiance makes for a very dangerous phase as she will eventually start to do massive amounts of damage. She will need to die somewhat quickly, especially if you ended Lu'lin's phase by killing her.
Save Cloak of Shadows for her Nuclear Inferno ability as this dealt roughly 50,000 fire damage per tick. It won't last the full duration so you will need to soak some hits. This can and should be Feinted.
It is important to note that the more damage you deal to Suen during the Darkness Falls phase the easier the Daylight phase will be but, bear in mind the consequences of having a longer Darkness phase as the Lurker's in the Night can also be just as devastating as Suen's Fan of Flames+Blazing Radiance combo.
Note: If you failed to kill Lu'lin in the first Darkness phase meaning that she is still active during the Daylight phase use her Ice Comet ability to your advantage. If Suen is placed near an Ice Comet her Blazing Radiance stacks will be reduced periodically. Additionally, you can also hide behind a Ice Comet during her Nuclear Inferno ability which will reduce all fire damage taken by 80%.
I did not attempt to use the Celestial Statues but if you can manage to use the Xuen constellation it could potentially allow you to avoid some incoming damage for a short period.