Water Conflux Plane Guide (LV 21+) by Nether

(this guide is brought to you by Nether (EU-47))
If you did just fine with Gelu up until level 20 on the Water Conflux Plane (check my previous guide for this), you might have noticed that after this level the setup just won’t work anymore. Don’t worry, however, it's possible to move forward, we just have to change the strategy.
=Motivation=
The Frost Lord’s attacks became much more powerful and it can easily wipe your melee damage dealers if you cast Frenzy on them.
The boss's ultimate skill, which she casts every 18-20s still one-shots your Ranged and Caster units.
Most noticeably, the melee burn/bleed won’t be enough to bring her shields down as she now has an aura that reduces their damage over time considerably.
These are the reasons why after level 20 we should change tactics.
=Hero=
We are going to use Mephala from now on. At this point this shouldn't be a problem as she is easily obtainable from the Main Story (and by the time you unlock level 21 you most certainly finished this stage). She is an overall great hero in PvE (with applications in other modes such as Endless Inferno), easily obtainable from the Guild Store. However, don't worry you if you haven't maximized her skills, you won't need to do that to pass the stage.
Of course, getting her to 4* and investing in her ultimate (Holy Shield) does make matters easier and requires less precision, so it's something you can take into consideration.
=Formation=
The most important unit to include into your formation is going to be Cyclops. It does tremendous burn damage whenever he casts his ultimate (Flamestone) skill, and that’s going to be just enough for him to bring the Boss’s shield down single-handedly. For some reason it works extremely well against the elemental.
* Healers: Angel, Genie, Monk, Pit Fiend
I run genie in my composition, but I’m well aware that it is not a free to play friendly unit. Awakened Pit Lord will compensate for some of that and Zealot awakening is coming as well next week which should further support the formation. 
* Damage-dealers: Berserker, Hydra, Red Dragon, Devil etc.
Here it's up to you to find the best damage dealers that do the job for you. Whatever strong melee damage dealers you have you will find will get the job done. I personally find Berserker to be universally useful thanks to his % damage on normal attack.
An alternative here is to go similar to Dragon Invasion if you have strong Grand Elf, Silver Pegasus, Marksman etc. With Path of an Assassin on them (purple work too) they can get also get the job done. This is riskier though as you can't spread them around too much so that Mephala's shield protects them all.
* Tank: Whatever strong defense unit you have.
You will have to put at least 1 strong tank in front of the boss so its normal attack won't erase your top damage dealers.
* Debuffers: Bone Dragon, Manticore, Unicorn etc. (or other debuffers)
Reducing the Unit Damage Reduction of the Frost Lord will increase your DPS, and therefore bring the Boss into its shield state faster. This way, the boss won’t have enough time to kill your Defense units.
If you really want to, you can also employ Darkness Enlightenment emblem set in the same direction.
=Strategy=
Unlike the Gelu strategy where just adjusting intuitively and casting Frenzy, timing is going to be the most important thing here. For this, I strongly recommend you play the battle at 1x speed (it's more tedious but it helps with precision).
The boss is going to cast her first ultimate around 10s into battle (2:50). You need to wait for the animation to begin and cast
Mephala’s ultimate simultaneously on your backline units, as to minimize the aftermath freeze damage duration. Here, note that the higher Holy Shield's level is, the longer it lasts, and therefore the less will the freeze damage affect your backline. 
Holy Shield's cooldown cycle is exactly the same as the boss' ultimate, so your melee units are going to take care of the in-between shield states.
Cyclops's ultimate is going to trigger sooner or later once the Boss is in her shield form and it should bring it down in 1-2s so that your dps can continue damaging the boss.
You can always use your other 2 shield spells on the frontline units, especially if you have leveled those spells up. Another application if you have 4* Mephala is to cast Stone Skin on your backline units to remove the freeze and eliminate the damage, but if you notice Holy Shield protects them long enough this is not necessary or useful. 
Repeat this cycle and you should be able to pass the stages without too much effort. Give it a bit of tries however to get used to the timing - don't get frustrated, at first it will be weird.
=Video=
I’ve also made a video on this topic where my formation and the process is visible, which you may find here: .
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