You're right 
She's awful! After kicking my arse up and down the dungeon she made me shake hands and apologize before getting my arm up my back and forcing me to eat the biggest, hairiest spider in the house. Jeez, I just wanted to turn her into a book 
I've read the mistress doesn't get good until later levels and I'd agree that seems to be the case - whipping multiple enemies is all well and good but the individual attacks are weak. I didn't know etoile skills activated immediately - TBH the whole maid/bard/singer thing confuses me, people talk about maids being essential.. I'm not sure for what though? I've never used the valkyrie (apparently they're vital) and can't get my head around dark lords either.. I dunno, at this point I'm feeling pretty beat up by it.
I just got to Chapter 8, so maybe I can offer some insight? The boss at the end of Chapter 7 went pretty smoothly for me so I feel like I have a better grasp on things than when I first started playing.
Maids - I find having one Maid very useful. Level up Generous Heart a fair amount because it will save you from having to use items to restore your team. My Witch is able to use one of her nuke spells to mop up trash mobs and, when then the battle ends, Generous Heart restores all of her lost TP.
I also recommend leveling up Maid Mastery a few levels, and then putting anything extra into Cook.
The only thing I like about Bards is that they get a song that increases experience. However, I find the Dancer class to be much more useful. Bards require casting time whereas Dancers activate their skills from the start. In the most recent boss battle, my Etoile lowered her speed AND attack. It made things so much smoother.
Of the two Maids you have, Ist is the better of the two because her Unique Skill ups the potential of all Maid abilities. Generous Heart will feel even more generous. One more thing about Generous Heart - you can run away from battle and it will still activate.
Valkyrie - I don't think that they're vital, but mine has proven to be supremely useful when fighting a single, difficult enemy because of one skill: Daedalus Lance. This attack hits 3 to 5 times at random. But, if there's only one enemy then it'll feel the brunt of that attack. Valkyrie's can also protect the entire team from magic or physical attacks if they have a shield. They're a solid tank, but when they don't need to protect anyone, you can use Daedalus Lance.
Dark Lords - I look at them as the Warrior class glass cannon. Two handed axes means that they can't use shields and they're slow as a snail. They make up for everything in raw power. Megaton Press can clean a battle in one use. Power Charge will increase their attack power for a turn to an insane degree. Grudge can restore TP since they burn through it quickly. Finally, they have a passive skill called Dark Quest. If your Dark Lord gets inflicted with any status ailment, then their power increases massively. The more inflictions the better. Aside from dangerous ones like Sleep, you can walk across traps to get her inflicted and watch the chaos unfold.
