The 1858 Wright edition introduced again right before against, an obvious dittography. This verse describes the first time that the people of Limhi decided to offensively attack the Lamanites (previously, they had been attacked by the Lamanites). A later passage in this chapter clearly refers to the second and third times that the people of Limhi attacked the Lamanites:
Thus the intrusive again in verse 6 of the 1858 RLDS edition is definitely an error; the subsequent 1874 RLDS edition removed the incorrect again.
Summary: Maintain in Mosiah 21:6 the phrase “to go against them to battle”—that is, without again preceding against; in Mosiah 21:6–10 we have a description of the first offensive attack by the people of Limhi against the Lamanites, which makes the use of again inappropriate.