mud-slinging match; smear campaign
Refers to a heated exchange of insults or slander, often in politics or public disputes. The term implies a back-and-forth, like a battle of defamation.
The election campaign has taken on the appearance of a mud-slinging match.
It's unpleasant to watch mud-slinging matches online.
誹謗中傷 is the general act of slander or defamation, while 中傷合戦 specifically emphasizes a mutual exchange or 'battle' of insults.
Compound of 中傷 (slander) and 合戦 (battle, fight). The combination creates the image of a 'slander battle' or exchange of insults.