I had this problem too the first time I played, because I just tried to ignore Harrow because I did not want Glory to kill him. I wanted to redeem Glory.

You have to keep killing Harrow everytime he spawns. When Harrow disappears he will not reappear for the rest of the turn or the next turn, but will appear again on the second turn.

You should be able to kill Harrow in the turn he spawns every time by focusing all attacks on him. So, it would be kill Harrow one turn, damage spirit on next turn, kill Harrow next turn, damage spirit the turn after that, until the spirit has no health left. In this way Harrow never gets to act and cannot heal the spirit.

If your character is not combat oriented, then this might be difficult because I find that Glory's accuracy sucks and it is hard just to use her to kill Harrow in a single turn.