It's not you or your team, that final boss just has a really annoying mechanic! xD To the point that I even avoid fighting her because lazy. TwT

Here's a quote pulled from another thread on how to beat her.

Link: https://www.sylestia.com/forums/?thread=18201&page=1#8 Author: Luga Time Posted: 11/30/2014 at 6:34 AM Here are some things that I usually do before I fight her. 1. Fight against mobs in the Aryth Spire until all of my pets' HP (at least 95%) and mana are full (If your pets aren't strong enough, you may have difficulty to defeat mobs in Aryth Spire so it might be better to fight mobs in lower regions). 2. After my pets' HP and mana are full, I will keep retreating from the enemies until I find a magical bottle which will give me a buff. Note that the magical bottle doesn't always give me a buff. If I'm unlucky, it will explode and reduce 40% of all my pets' HP. When that happens, I'll fight the mobs again until my pets' HP and mana are full again and repeating the process of finding a magical bottle that gives me a buff. 3. After I get the buff, I will go to Dungeon of Forsaken to fight Vierna. Here are some tips to beat the Vierna. 1. Use finishing blow [hotkey F] immediately in your 1st turn. 2. Make sure you don't attack Vierna's minions because killing them will just increase Vierna's power. 3. After you beat her, she will change to her final form, Shade of Vierna. Now is when you really need to count your turn. Don't see the turns on the battle screen! It can be misleading. In 1st-4th turn, just attack her with hotkey 1 and use finishing blow in 5th turn. Pay attention to the attack log (below the battle screen). If you see "Mana Transfusion" on the attack log, that is your 1st turn mark. So you'll only have three more turns to attack her with hotkey 1 before you use finishing blow in 5th turn again. If you don't see "Mana Transfusion" on the attack log, she will most likely uses it on the next turn. Just remember that "Mana Transfusion" is your 1st mark and you'll be okay. :) 4. It's best to heal your pets after she uses Mana transfusion but you can also heal your pets if you haven't reached 6th turn. If you still can't beat her, there are probably something wrong with your set up. If that is the case, you can read team set up guide by Divinis here. I also put my team set up there, but my set up are spesifically for fighting multiple enemies so Divinis's team set up might be more useful than mine if you're planning to fight single enemy.

Oh thank you!
I hadn't even thought about going back to find the buff bottle.
I'll have to do that o':