Lemme see here...
Youmu's Ghost Half: takes 130% damage from SPI; 50% damage from CLD, 80% damage from MYS. MND is half of DEF. Chance for PAR is only 1/4, but debuff chance is 1/2; favour trying to drop its speed first.
Youmu herself: No elemental changes. MND is half of DEF. Chance for PAR is 1/2, and debuff is 3/4. She's easier to disable than the ghost half as a result.
Also, you still only have a limited party right now (Don't forget to go get Minoriko if you haven't yet!). Don't flat out drop Sakuya or anything - keep her in and let her swap in for a more key character that needs to recharge SP, and have her throw down a Lunar Dial in that time. Youmu's got a lot of speed for where you are in the game (125 or something like that), so every bit of advantage is still a bit of an advantage.
Finally, Meiling might seem to be a better choice for a 'main' party than Remilia, but I'd only say to keep Meiling in your party for bosses. Her ability to handle random encounters drops really fast - Remilia shares the single-target limit, but she tends to be capable of doing far more actual damage output.
Good luck out there.
ALSO: to everyone who has ever suggested that the v2 bosses are a cakewalk, I want to hunt you down. The damage these bastards do is ridiculous.