According to the regiment’s record of events, the 18th left Alexandria on May 2, 1863 and arrived on the 12th of May at a camp near Nachitoches. Seems out of the way to have been in Louisville (Gansville) especially that quickly. He may have been part of a scouting or foraging party ahead of the regiment but it would seem it was probably not a skirmish or battle with the whole regiment. Have you looked to see if you could find any others from the 18th that were captured, wounded or killed on the same day?

I questioned my brother who was a pastor in Gansville. His reply doesn’t answer your question but adds some points of interest.

“Well, Louisville is now Gansville, northwest of Dodson where I served
my first pastorate at Siloam BC. After the war, a carpetbagger named
Ganns came into town and built a cotton gin and renamed the town after
himself. Evidently, Louisville was a stopover on the ride from
Monroe to Natchitoches, and the story I heard that the road out of
Monroe was the road to 'Louisville'. The Baptist church dates from
the 1850's. There are 18 or 19 Confederate veterans buried there, not
from battle--they were residents who lived out their years there. When
I lived there, the only thing I could find was that there might have
been some skirmishes between outlaws and draft dodgers and the troops
the Confederates sent to round them up. Be glad to hear if you find
out anything else. If you look at your map, La126(I believe) leaves US
167 at Wyatt, goes to Gansville, then La 1234 bends back to Dodson and
rejoins US167. I think that was the original road to Pineville/Alex. I
did read that the La Brigade marched up that road at least once to
Monroe, at least the 18th did, as locals walked with their loved ones
part of the way and brought food to them.
One of the last things I did as a chaplain in the La National Guard, I
got the VA to mark 12 of the CSA graves with the bronze veterans
markers. They had family markers but no reference to civil war
service.”