Hi bros I'm Niko, today I will explain about Arcane and Mount Skill - how it work. This is for the best damage you can make to a target. 

First, let's start with the definition of Arcane.

As the picture above, you can see there is 3 types of Arcane: Infernal (orange), Glacial (blue) and Vital (green). 

The Infernal overpowers the Vital, the Vital overpowers the Glacial and the Glacial overpowers the Infernal (as the red arrows I show you)

Overpower here means: for example, you cast Vital while your opponent casts Glacial, that makes you dominate the battle (your damage to the opponent is higher than usual).

An assume for this: if you and your opponent choose Infernal (same Arcane) your damage to him/her is 1k and also him/her damages you with 1k. But if your opponent choose Infernal and you choose Glacial (you dominate) your damage to him/her is 1k5 and him/her damage to you is 800. (don't remember exactly it also affects the Def or not).

Next, I will show you the Arcane of Eidolons:

If you want to cast specific skill of an Eidolon, you have to know which type of Arcane that Eidolon use. Type of Arcane of Eidolon is the final Arcane on the row(but in battle it is final Arcane of the column). I call this Arcane-use.

I make an example with Athena, Veiled Nemesis and Mutant Sigurd

You can see Athena has 1 Infernal and 1 Glacial.

Athena's Arcane-use is Glacial (the last Arcane in the row), in order to cast her Guardian Clone, you have to choose Glacial, but first you have to choose Infernal (Athena does a normal attack when you choose this). That is because in order to use Guardian Clone, she has to consume Infernal first. 

Eidolons consume Arcane from top to bottom, that is the reason you have to choose Infernal first.

You can see her first circle turns into None when I choose Infernal, means the Infernal has been consumed. Now we are ready to cast her Arcane-use.

After that I choose Glacial, and she casts Guardian Clone. Then she is back to her start (having 1 Infernal and 1 Glacial)

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Next is Veiled Nemesis and Mutant Sigurd.

Veiled Nemesis, her Arcane-use is Vital.

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And Arcane-use of Mutant Sigurd is Glacial.

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As I mentioned above, there are total of 3 Arcanes, but now we have the Grey Arcane? the Grey Arcane is actually a variable Arcane, that means you can choose either Infernal, Glacial or Vital.

In this case, we have to put them into a battle:

The first Arcane has been consumed (choose either Infernal, Glacial or Vital to consume)

Mutant Sigurd needs only 1 Grey Arcane, but Veiled Nemesis needs 2, so that now we can cast Mutant Sigurd Arcane-use, but for Nemesis we have to wait one turn more.

Mutant Sigurd's Arcane-use casted.

Also Veiled Nemesis's second Grey Arcane has been consumed, so now we can cast her Arcane-use.

Mutant Sigurd is back to his row (1 Grey Arcane and 1 Glacial)

Veiled Nemesis's Arcane-use casted. And Mutant's Sigurd takes it as Grey Arcane.

So now you understand how it works with Arcane and the specific skills of Eidolons. 

The final thing is showing you the Mount Skill:

I choose an event for this, I forget the event name but the opponent of this event is Gilgamesh

If you notice his Arcane, you will know he will use Vital => Glacial => Infernal. So that in order to beat him, you will use Infernal => Vital => Glacial to take the dominant and makes higher damage to him.

First I make Vital:

https://prnt.sc/fry8q3

[access the link above to see the screenshot, the size is too big that's why I have to use Lightshot]

For once you dominate the battle, the skill bar will increase to half (skill recharge to half). For 2 times dominate the battle, your hero can cast mount skill. 

That is all for my explaination. Hope you bros enjoy it.

*Notes:

When you are in Arena, your rival is on AFK mode, so that choosing the right Arcane is the most important to win the battle.

If you choose wrong Arcane, your opponent will dominate the battle and she/he gains skill recharge.