Here's some tips:

(SPOILER)Ignore all manner of machine gun scout, machine gun spider, and 'Battlecopter' robot enemies. These were the scum of the earth during the main game, so there's no reason not to think so now that they're on steroids.

Save every few floors. There's nothing worse then climbing ten floors only to be anhillated by uber-monsters. Most of the important items (new equipment and skills) will be near the exits and entrances of new floors, so the only thing you'll have to actually search for are crappy Jewel items and the much more important stat increasing berries.

The bosses should all be easy up until the Arch Demon. Super Blair and the other returned Sphere Employees should be moderately difficult, but not too much. Remember Eddy of Light is now a major attack, so the only way to avoid it is to sidestep back from it twice.

Lenneth (from Valkyrie Profile) will be a challenge. Expect to die many many times before conquering her. Remember, never let her get off her Major attack or Nibelung Valesti. This will mean certain doom for anyone caught in their range of attack. Stun her with any damaging bomb or item to interrupt her, and keep one character on the opposite corner of the battlefield to heal and support the party (Peppita, Sophia, Adray, and so on). But she's so cute, so it'll be hard to see her go. Remember to make a save before her so you can get the Battle Trophies from her in the future.

Ethereal Queen will be a challenge, but if you can beat Lenneth, you can beat Ethy. You can fight her as many times as you want for Battle Trophies.

The real challenge is Freya (also a chibi-fied cameo from Valkyrie Profile). After defeating Ethereal Queen, you can return to Crosell's chamber and fight her (Crosell and Albel if you didn't recruit him will be gone, so farm Boots of Prowess before you defeat Ethy for the first time). Go up to Crosell's altar and choose to put the orb down (save in the nearby bonus dungeon, the Urssa Cave Temple. There's some more bosses there, too).

Freya is horrible. I've only beaten her a few times on Galaxy mode. She has 20,000,000 MP and some other godawfully high MP total. The same strategy for Lenneth applies to Freya, but the only difference is that it'll take so much longer, and if you mess up once, it's all over. Ether Strike nearly envelops the entire arena with supposedly purifying energy, dealing extreme HP (~30,000) and MP damage to everyone caught in the blast, and deals paralysis. Her normal attacks cause confusion, and if you strike her from behind she automatically counterattacks in a spiral around herself. Only the toughest defense melee characters (usually with several Boots of Prowess equipped) can stand up to her attacks, and Ether Strike just means death regardless. However, defeating Freya provides a wonderful sense of achievement (or perhaps, a dishearting realization of far too many hours spent playing this game).

Yeah, anyway, good luck. That was probably too much typing, but yeah.