“Who are you supposed to be? Playing the heroes in a foolish knightly romance? You may think you are helping the children, but you're only rushing to your doom!”
Heintz is the boss of chapter 7 of Fire Emblem: Rekka no Ken. According to the inkeeper who kicked Nils out of the inn, Heintz worked for the Black Fang and his mission was to capture Nils and Ninian. He succeeded at capturing Ninian. However, Nils managed to escape and met up with Lyn and company. Her team came to his aid and defeated Heintz. Later on, it is revealed that Ninian was rescued by Eliwood.
Dark - D
The best way to beat Heintz is taking advantage of his low defense by attacking him with one of your stronger units which should be protected with the Pure Water available at the village. Lucius is also a great choice to weaken Heintz or to give him the final blow, as he would not affect him much. Also, he is not that fast, so a quick unit can bring him down easily.
It is tempting to train Lucius by taking advantage of Heintz's healing after each turn, but it is better to defeat him quickly because of the sidequest you can take if you beat him in 15 turns or less.
“You were only...striking at air...You are...too late. The girl is already--”
—Heintz's death quote