I recommend you to use Elwyen along with Zach. Her Ballad of Ballance enables him to attack all enemies at once. It’ll be really effective especially when you’re in Tained Ghetto, since his soul element is air and most of the monsters’ element are earth.
Also, this will give you many battles, so you can learn many passive skills while making Sin Strike stronger.
Tip: My Zach (Lv 31) deals about 1.1k damages on Furry Biter (0DF, 0% resist dark). Also, on Chrysophage (0DF, -100% resist dark), he deals about 5k damage.
Hope my comment is useful for you. Good Luck ;)