I just now ended up defeating the Hydrangeas by using the Nasty Weapon Coating (or whatever it was called, gives +4 to ATK for a day) and a Vitality Potion. I'm not sure if this was a worthwhile trade - I think it'd be more cost-effective to go with your idea with the bombs.
I've been uncertain of where to use Pierce since the game doesn't usually tell me how heavily armored opponents are, so I've hesitated on spending the two days to get it. I suppose it'd be useful against the demon.
Spoilers and all that, but there's a boss with 55 DEF you have to get through (on your own if you don't go the lewd route) - you'll know him when you see him. Pierce, IMO, is a lot more viable than Brittle for that battle. Not only that, but as just about all enemies have some measure of DEF, you're going to do more damage with Pierce than you would with any standard attack. Sometimes it's not necessary, but the extra damage may save you a round or two of fighting or could even be the difference between dealing damage and not.
