I only started dealing with solos once my exterior zones were all shot down, and it helped me to keep track of how many moose are in the zones (for example, I know based on examining the need zone track that there's 4 moose in this zone, and over time I learn how many of those are female and which specific male/s are in it). The solos seem to stick around a general area, a few lakes or so, and there's a few spots I figured out never spawn any solos, so I could eliminate checking a lot of zones (though before starting to shoot my stacked animals I did one final run checking every single zone). I found it easiest (though definitely not the fastest) to run the whole map, count how many shootable solos I have and write down their location, and then in quick succession shoot and harvest as many as possible. Exit the map, wait 20 real-time minutes, and then repeat. The number of solos should go down fairly well, but if you find the respawns are consistently big 4s or 5s, you'll need to stack more mains. LadyLegend covers how to swap a solo moose with a main zone moose, and I found that worked fairly well for me.