Special atack

It's just a normal damage buff but applied against enemies with specific traits.

Special Attack is like a Hard counter only designed to specifically beat the targeted enemy.

For example Siegfried Laughing while he fight Artorias, Liz, Kiyohime and other Dragon.
Also with his skill upgraded, pretty much a walk on the park.
His Special Attack Gives 80% Bonus damage to Dragon + 50% Buster damage + 30 % Defense against them.

Nightingale also is a Supp with Hardhitting Special Attack for Humanoid enemy.
Bonus 50% damage and 25 % damage reduce is considered strong. Also come with targetable 50% Buster buff. Don't be tricked by her Costume, she's a Monster when fighting servant also she's a Berserker.

The only weakness for Special Attack is the Niche, which is not recommended to use for New Master and for anyone new to the game.

And because of their Special Skill Attack, seasoned player use them for Meme reason.
Such us Tanking Specific servant NP and killing them for just 1-2 turn and 3 turn is the longest average.