King Limhi is here saying that after his people’s earlier, but unsuccessful, strugglings (or struggles) to escape from the Lamanites, there remains one more struggle to be made—and he believes that this struggle will succeed, unlike their previous ones. The word effectual is now unusual in commonly spoken English (although there are some uses in more recent times in the law and in economics, according to the Oxford English Dictionary). The OED explains that earlier the adjective effectual had a more general application and could be used to refer to anything ‘that produces its intended effect’ (see definition 1), as in the following citations:
Thus there is no error in the use of effectual in Mosiah 7:18, even though the phraseology may seem odd to most modern readers. It is doubtful that the word is an error for the alternative adjective effective.
Summary: Maintain the use of the word effectual in Mosiah 7:18; its nontechnical use in the noun phrase “an effectual struggle” can be supported from earlier usage in English.