'sole method' is correct and usable in written English.
You can use it when you are emphasizing the only way of doing something. For example: "For the given task, the sole method of analysis is a statistical one."
After that, ratification is the sole method for a government to express support for the treaty or associate with it.
According to Havel, "the sole method of politics is quantifiable success," which meant that "good and evil" were losing "all absolute meaning".
(Nebraska was the last U.S. state to use electrocution as the sole method of execution; in 2008 the Nebraska Supreme Court ruled it unconstitutional).
By 1999, 34 of the 38 states with capital punishment had moved to lethal injection as the sole method or as an option, largely because it was considered more humane than the other methods.
In politics, good and evil, categories of the natural world and therefore obsolete remnants of the past, lose all absolute meaning; the sole method of politics is quantifiable success.
Georgia's Supreme Court became the first appellate court in the country today to rule that electrocution is an unconstitutionally cruel and unusual punishment, leaving Alabama and Nebraska as the only states using the electric chair as their sole method of execution.
It taps the least exciting/most frustrating part of the game and makes it the sole method of catching these guys.
It's not quite ready to be a sole method of release, but it's really just a matter of time before that comes to pass, too.
Nothing should stop the historian from making such comparisons as long as it is not his or her sole method of analysis.
Thus, fusion imaging cannot be used as the sole method of guidance, and US should always be considered the reference for device insertion.
Histological evaluation remains the sole method of distinguishing steatosis from advanced forms of NAFLD, i.e. non-alcoholic steatohepatitis (NASH) and for the evaluation of hepatic fibrosis [77].
