The phrase "joining battle with" can convey a sense of conflict or opposition. However, there are many antonyms that denote collaboration or cooperation. Words such as unite, align, ally, unite, or team up connote joining forces for a common purpose. Collaborating, cooperating, or working together are other antonyms that suggest a working relationship that is not adversarial. In contrast, antagonizing, fighting, or confronting are further antonyms that signify a contentious or confrontational relationship. Ultimately, the choice of antonym for "joining battle with" depends on the context and intention of the speaker or writer.