What factors influence the likelihood of item drops in Recettear?
Solution 1:
Chaining Kills only appears to affect Experience Gems.
"Stronger" monsters have separate drops because they are separate monsters. A level 10 green slime and a level 1 green slime use the same drop table. But a Level 60 Grey Slime can drop different items than either Green Slime.
Thus, if you are trying to farm a particular material, it is advisable to go to the first / most common appearance of that monster. Difficulty is irrelevant when it comes to item drops.