Heya friend. Why don't you post your team and your roster, and we'll see if we can help you get a strategy together.

Here's the very F2P-friendly team I used last rotation.

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The most common strategy is to bring a lot of turnmeter control and at least 1 strong nuker that can wipe the mirror copy of your team in one shot. 

Also make sure that your first turn meter control (aoe) champion is faster than the boss. All of of your other champions are speed tune to move next after your first turn meter control champion. 

Lastly, don't bring tanky champions. Some players even remove some of their champion's gear to be squishy so you can one shot the mirroy copy of your team. 

Dark Fae is one of the easier Doom Tower bosses in my opinion. 

In my case I use a very squishy but hard hitting team with turn meter control. Please find below my Floor 40 team on normal.

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Obviously High Khatun has the first turn and boosts the turnmeter of the entire team. Mirror High Khatun does the same. Afterwards my Warmaiden uses her A3. As my Warmaiden has high accuracy but has almost no defence and hp the mirror version dies instantly. 

In the next step my Soulbond Bowyer uses her A1 and severly damages the mirror teams, except normally for Alure and Elhain. Also Soulbond Bowyer normally kills her mirror version.

Then Elhain enters the stage and beats the remaining champions of the mirror team. Finally my Alure gets a turn and uses her famous A1 to decrease the turnmeter of the boss.

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Finally beat  her  with  this  team  thanks!!

This  is  my  team  i  use  septuims  and  cupides  are  for  nuking  her  copys  and  the  others  are  just  to  get  me  to  her  after  the  copys  are  gone  its  strught  forawrd  

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Congrats! :D :D