Did you really manage to do this???
Yes, got him to level 20 via dodge tanking for thousands of turns. Used him as a dodge tank for Chapter 2 to safely bait enemies into my range. Promoted him on Chapter 3 and just let him fight everything he could reach, and just let him deal with the fliers in Chapter 5. Then I did his event Chapter 6.
I was understandably terrified that I wouldn't be able to beat the last chapter of this version.