LDS apologist Dennis D. Chamberlain offers this succinct answer to the claim 2 Nephi 25:23 contradicts Eph 2:8-9:
In Eph. 2:9, Paul was writing to baptized members of the church, admonishing them to live by their faith in Jesus Christ. With this understanding, Paul’s statement does not contradict the following familiar Book of Mormon verse: “For we know that it is by grace that we are saved, after all we can do.” (2 Nephi 25:23)
Nephi does not tell us that we are saved by our works. Nephi tells us that we should reconcile ourselves to the will of God, but then after we are reconciled unto God, we are saved only by his grace:
“Wherefore, my beloved brethren, reconcile yourselves to the will of God, and not to the will of the devil and the flesh; and remember, after ye are reconciled unto God, that it is only in and through the grace of God that we are saved.” (2 Nephi 10:24) (Dennis D. Chamberlain, The Biblical Evidence of the Restoration [Millennial Mind Publishing, 2004], 277 n. 1)