Here the typesetter for the 1837 edition introduced come into the text, an obvious typo for came. The rest of the passage reads in the past tense. Amazingly, this reading was maintained in the printed editions until the 1892 RLDS edition (for the RLDS text) and the 1911 LDS edition (for the LDS text). It appears that earlier editors and typesetters were willing to interpret this case of come as an instance of the historical present tense.
Summary: Maintain in Mormon 6:8 the past-tense came in “they came to battle against us”; as is appropriate, the entire sentence is in the past tense.