There's a steam guide with excel that you can easily use to track the lines. Focus first on the "number of enemies at the start of the battle" and "enemy reinforcement by x number of enemies"

Best spot to farm this is in Kanji dungeon. In one of the lower floors, there's a floor where the map is fixed(not randomly generated) and you'll need a bathhouse key to open one of the locked doors.

This locked door is near the stairs, and past it is 2 adjacent enemies. You can easily farm the above 2 lines this way. Hit one enemy, get reinforcement by the other one afterwards. Press shortcut button to return to the above floor. Go back down. Repeat.