I paid attention to this now that you said it, and truly, they hurt a lot more than I expected. I can just entirely ignore them thanks to double Jack Frosts, elemental advantage and a blanket -40% Atk down. Without those, yeah, they'd hurt. Well, I forgo the -20% Fire Res for double Reiki on Sundays, and I can still ignore them, but you get the idea!
Not sure how I'd rank the trash waves then...It isn't? Because they don't hurt much if they're not buffed, if you ask me. Yes, it takes BLOODY FOREVER to kill them when you wait 3 minutes after every few turns, as without Def debuffs you won't do much damage during those few turns. But as long as there's two of them buffed, I'd consider letting them attack - even when Atk debuffed - suicide.
I guess if you can land -60% on both and combo-, then you're dealing with 140% damage, compared to 100% if you'd wait? That may be worth it for the extra damage you deal on them. If your healers can handle the extra load.