I'm not one of the game's Combat experts, but if you're going to use Auto-fight, it seems to me (manual Combat player), from much time spent watching how the AI behaves, that the AI [at time of writing] assumes the use by Players of mainly/only Mercenary Camp Units, probably at 3* level.
This means that the AI will usually place other types of Player Units, especially Barracks Units (generally higher HP but also generally lower damage), into positions where they are more readily killed than their broadly equivalent Merc. Camp Units, with the different move/strike range of pre-Merc. Camp Mages - which are often the Unit on which any given battle hinges - being of special note here (NB : excluding Human-only Priests, which have a move 2 / strike 5 range). Conversely, it's also possible, where fighting Manually in particular, to 'trick' the AI by intentionally using the theoretically less-versatile move/strike range of Barracks Units in particular, plus [rarely] Cerberus Units, to one's advantage, since the AI defends poorly against them - again because, in my opinion, the AI understands primarily, if not solely, how to use and/or defend against Merc. Camp Units.
I generally would not use Barracks Troops - other than Priests if playing as Human - in Auto-fight Combat against non-ideal opposing Enemy Units, especially where the battlefield Map obstacles don't favour their move/strike ranges, although in some cases Golems, if playing as Elf, can be very advantageous, especially when not combined with other Units - assuming that the nature of opposing Enemy Units is suitable (few or no HM).
The * rating of Units, both Player and AI-controlled, is very significant in terms of debuffs - I am fairly confident that the AI assumes all-3*, always.
Scouting the battlefield before Auto-fight is important in ANY battle and can be vital, especially using Barracks Units, and Manual Combat is safest.