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Attack.
Assail means to attack violently and persistently.
attack, assault, besiege, beset, assail, bomb, shell, blitz, open fire
assault.
accuse, indict, impeach, incriminate, arraign, attack, assail, rush, storm, stampede
The root words for "assailant" are "assail," meaning to attack or assault aggressively, and the suffix "-ant," which is used to indicate someone who performs an action or holds a particular quality.