Only foods up to the column "Food 3" drop from enemies. Same applies to the Pet Monitor. "Food 4" items may be found in pots or chests, but not all of them and not enough for exchange. So the goal is to get up to 100 of a "Food 3" item, which gives you 10 of the corresponding "Food 4" item which, when going for their exchange, gets you a reward (and you get to keep those food items as well). You can only obtain the reward once per playthrough.

In order to reset a level you need to leave the area entirely. This means either going back to the map or warping to another area and coming back. For example to reset the Serpentina level Path to Lair you need to exit Serpentina AND the Serpent Ruins, so you need to warp elsewhere (Puck Village works).

Look in your bestiary what enemy or enemies you need to kill. Make sure the food you want is the first drop listed in there as it'll drop far more often (the last food listed is the rarest drop). Only focus on killing the relevant enemy or enemies. Reset the level haflway through if there's few opportunities to get the enemy you want.

Don't forget that when you can warp around you can enter levels from the end. For example you can warp to the Serpentina Lair area and go to the Path to Lair level from the 4th area by backtracking. This allows easy access to some enemies that drop the food you want, in this case Oni Samurai which drops Muscat Bunch.

Focus on food in the order given in this list as you will get rare drops and rare drops are almost always better food.

Enjoy.