Here “soldier” is comparing a person who fights physical battles with a believer who fights spiritual battles. Paul is emphasizing that a believer has to struggle against evil in order to spread the gospel. This could be translated as "my fellow believer who works and struggles along with us." ([[rc://en/ta/man/translate/figs-metaphor]])
## and your messenger and minister to my need ##
"and who brings your messages to me and helps when I am in need"
## he was deeply distressed and longed to be with you all ##
"he was very worried and wanted to be with you all"
## sorrow upon sorrow ##
The full meaning of this phrase can be made explicit: "sorrow added to the sorrow I already have from being in prison." (See: [[rc://en/ta/man/translate/figs-explicit]])