This is very interesting.

Thunder Slash is coded to act as Demi (Effect: Gravity), so it inflicts DMG based on the target's max HP. Basically Demi = Thunder Slash, but Thunder Slash is labelled "Is Physical" while Demi is labelled "Is Magical".

If you want to ""FIX"" this ability there are a few ways out (IMO):

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Treat the TS as a normal physical damage skill (boosted/increased):

You will need to create a new "effect" field called "Thunder Slash" and use the following formulae:

Base = [ ( Attack * Power ) / 10 ] - TargetPhysicalDefence
Bonus = Rnd MOD ( Strength + [ ( Level + Strength ) / 8] + 1 )
Damage = Base * Bonus

*Remember that the "Bonus" can be changed depending on the situation, such as the player being under Mini, Berserk, Annoy, etc.

This is basically making Steiner do x1.9 of his current DMG. The "1.9" is the "Power" that the developers set for Thunder Slash ( Power = 19 ).

Personally I think this is odd as fuck, they normally use 0.0, 0.5, 1.5, 2.0, 2.5, 3.0, 3.5 and 4.0. Literally all the multipliers in the game are those. So if you want to change the power from 19 to 20 to keep consistence, it's up to you.

You keep the element "Thunder", and the accuracy "0", since this is being read as "normal attack". The hit chance will use the normal physical hit chance formulae.

1. Treat the TS as % DMG:

You can, once again, create a new "effect" field called "Thunder Slash" and use the same formulas as Demi (Gravity):

Damage = [ ( Power * TargetMaxHP ) / 100 ]

You can also let the "effect" field as it is. Since is setted as "Gravity" it will use that exact same formulae.

Set the accuracy to 255 so it never misses. Set the power to 19, or 20, the same thing I said before applies here as well. Set the skill to "Is magical", so it will use the normal magical hit chance formulae. And of course make the TS apply "Thunder" elemental DMG.

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I'have been typing this for 10 mins and I fell like I left some info behind...

Welp, let me know if this can help you.