The part you have to hit is definitely tiny (you have to actually hit the ear and not anything else). Honestly there's no real reason to get Bat Ears anyway though; if you need dark resistance Curse Checks are a bit easier to find the parts for imo and if you need protection from frailty just use a Nectar Potion.
The only opinion one should care about is one's own. We did. You did. Yes we can. No.
How much HP does the ear actually have? And will instant breaking work? No doom resistance? I think I'll just resort to prism gem. This item along with prism crystal helped me obtaining Arondight here at Chapter 3. I just couldn't remember where I got the prism crystal though...
Instant breaking works ( Beast Bludgeon ). Enemy parts don't have status resistances that differ from those of the main body ( they may have different elemental resistances, though ).
The hard thing is, as is already mentioned, to actually hit the right part. Personally I've managed to get one Bat Ear in several hundreds of hours of gameplay ( and unfortunately I don't even remember what character and attacks I used ).
"C.Chalice - Dream or deceit ? Who will ever know ?"