This is not so much a solution as some assistance in doing this achievement's fights. Dolce is one of the toughest bosses in the game, and even the one mandatory battle with her is a toughie. Grind as much as you feel you need to, but I beat her on the second playthrough, Party Level 6, with nobody in my party higher than level 32. I was using Falsetto, Jazz and Viola.
Tips for fighting Dolce:
-you want as many harmony chains as you can get, party level 6 is invaluable. Therefore you need to build them as fast as possible, using fast-hitting characters like Salsa.
-fill the top part of your item list with Angel Trumpets, the bottom with Club/Diamond Clovers. When the fight starts, and you have a moment, scroll to the middle so you can quickly scroll up or down one position to the one you need to use. Put some Goddess Bouquets at the top or bottom of the list for when two of your three party members are downed.
-fight Dolce in the light, so your healing characters can use their healing spells.
-when the battle starts, retreat to the edge of the battlefield, or to an obstacle, and keep your back to it so Dolce and her lieutenants can't get behind you and score unblockable hits.
-keep your party together, with the healer at the back, so Dolce and her lieutenants all come close to you in a big clump. Then, when you attack, you'll hit more than one of them with each attack and increase your chains faster.
-kill the lieutenants first. They hit hard, and can resurrect each other. If you keep them all grouped, you should likely kill one within one or two turns of the other dying.
-once Dolce is on her own, surround her and pound the crap out of her. You should be getting a 24x or 32x harmony chain with each round.
A few things to bear in mind:
-you only get a 6x harmony chain if each character has two different special attacks of the appropriate type (light or dark) unlocked - they can't do the same one twice.
-if you have any items to recover your health or boost your chains, use them! Falsetto with the Werewolf choker can get a 24x multiplier all on her own if she's near an enemy.
Good luck! These strategies worked for me for all four battles.