Last Attack

A Last Attack (止めの一撃), is the final strike on rare monsters, like bosses or sub-boss-class monsters. A player who deals the Last Attack on monsters receives more experience and are more likely to obtain rare drops known as "Last Attack Bonus Rare Drop" rather than assists.

In Aincrad, all floors (except for Floor 100) are known to have LA bonus rare drops.

Reference
Último ataque Last Attack Последняя атака