"They are fighting Calum."
Translation:Tha iad a' sabaid ri Calum.
It could be translated to English both as they are fighting Calum and they are fighting with Calum (and I think both answers should be accepted); but see how the latter is ambiguous in English, it can mean either that they are fighting against him as well as alongside him (and I think the Gaelic a’ sabaid ri means rather the former) while fighting Calum unambiguously states that the fight is against him.
Also, in Gaelic sabaid is a noun, a fight (tha iad a’ sabaid ri means lit. they are at fight with/against…), so it cannot take a direct object, you have to translate English they are fighting Calum to tha iad a’ sabaid ri Calum.