1) Aggressor 2) Ambusher 3) Assaulter 4) Attacker 5) Avenger 6) Bludgeoner 7) Combatant 8) Harrier 9) Hooligan 10) Lapidator 11) Marauder 12) Nightrider 13) Opponent 14) Piranha 15) Predator 16) Raper 17) Rapist 18) Retaliator 19) Robber 20) Roughneck 21) Ruffian 22) Shedder 23) Slasher 24) Spiller 1) Aggressor 2) Assaulter 3) Attacker 4) Bushwhacker 5) Enemy 6) French word used in English 7) Invader 8) Mugger 9) Offender 10) One who attacks another 11) Someone who attacks 12) Thug at work 13) Wrongdoer 
• (a.) Assailing; attacking. • (n.) One who, or that which, assails, attacks, or assaults; an assailer.
As·sail'ant adjective [ French assaillant , present participle of assaillir .] Assailing; attacking. Milton. 
assailant 1. Someone who violently attacks somebody else, usually causing physical injury. 2. Someone who assaults, attacks, or invades with violence.
a person who attacks.