A combination of first setting or riding levels and then secondly having compressor/ limiters to even final levels whilst maintaining punch and perceived loudness. The Compressor limiters help smooth variation in projection, but the only way to make them work properly is to first get basic levels smoothed out.
btw, the plugins in FCP and the tool for riding levels are crude. STP or an external audio DAW will be a better place to do that. If you want to do it in FCP then settings for compressor and limiters have been discussed here many times and a search will reveal opinions on good settings. I am lucky to have a Fairlight system to OMF out to so I have never played with FCP's audio filters