Farming best done in campaign, Brutal difficulty, 12th (last) level, 3rd stage.  You'll see this as Brutal 12-3 or 12.3 depending.  This optimizes your energy expediture vs experience and silver gained.  Now understand this isn't really "food" per se.  You are using this stage to level up your uncommon champs to three star, then four star, then five star so that you can get your 6* worthy champs to max (except Armiger, don't sacrifice Armiger).  You're first farmer should be your starter.  Once you get them to 60, they should easily be able to handle anything in brutal by themselves.  Later on, you'll get better champs for this.  But start with your starter champ.  Use only your farmer and three uncommon champs (Chickens or food).

Get your starter champ to lead a team of food champs in Campaign 12-3. If your starter champ is only 5*, then do it on hard - if it's 6* then do it on brutal. 

Your food is any 1-2* champs you summon from mystery shards or pick up from campaign battles.

This all presumes you put your starter champ on the lifesteal artifacts you got at the start, to make them self-sustaining.

to add onto the prior posts, until you get later into the game:

The highest 3rd stage you can do (for silver/xp overlap due to shields) until Brutal 12-3.

The highest 6th stage you can do for pure xp, again until Brutal.