Character Skills have their own special properties,

their specific combination (key) unlocks the boss's protection.

Lahn Skills

Defeating the Level 20 boss in Trias of Ator - Elite
you need to use two "keys"

FIGHT -> only for combat support (optional skill)

1st key: Stun -> Daze -> Knockdown

2nd key: Daze -> Knockback -> Stun -> Knockback -> Knockdown

Trials of Ator Elite lv 20 -> Lahn Skills Setting

You can configure and save the appropriate settings for your character.

When prompted to unlock boss protection, simply activate the skills in the following order during the boss fight:

first yelow key (1 -> 2 -> 3)
Lahn Skills:  Blooming Slash ->  Briar of Thorns -> Four Point Splatter 

&   

second green key (1 -> 2 -> 3 -> 4 -> 5)

Lahn Skills:  Briar of Thorns -> Crescent Barrage -> Blooming Slash -> Crescent Barrage -> Four Point Splatter 

and the boss protection will be unlocked...

Trials of Ator Elite lv 15 -> Lahn Skills Setting

When prompted to unlock boss protection, simply activate the skills in the following order during the boss fight:

green key (1 -> 2 -> 5)
Lahn Skills:  Briar of Thorns -> Crescent Barrage -> Four Point Splatter 

and the boss protection will be unlocked...