Spoiler:

I will say this now, BUILD UP Minwu, Josef, and Ricard! I'm talking 1500+ HP, 300+ MP, level 12+ weapon (whatever you decide) and shield, and level 10+ Cure, Esuna, and Life. (You can buy those three in the added section, but best to build them up when given the chance.)

Everything except inventory carries over to Soul of Rebirth. So, you're gonna have to equip your characters as beefed up as possible. For Josef and Minwu, that's Mysidia's gear. For Ricard, that'll be whatever you find in the Whirlwind (Black Robe, Osmose Scroll if you are lucky enough and one drops.) Before he croaks, give him the Sun Sword and Defender! (I think I'll be killing much easier if I had these.)

Some of these enemies are quite annoying, and I'd love to have a final weapon to assist in killing them. So far I've come across an enemy with 5000 HP and high enough defense where I hit for pretty much nothing. I won, but it was a very sllllllllllllllllllllllllllloooooooooowwww battle. Endurance match anyone? :p This is why you want high level weapons, more hits. More hits, more damage. And unfortunately for me, Minwu is the only one with such stats. Ricard has ok stats, Josef was still in Leather gear...

Killing the enemies won't be hard to start out (even with two sub-par characters) but once you get to the next area......not so much.

There's two "dungeons" that I have seen so far. I'll dub them Mirror Jade and Mirror Pandemonium (that is, if someone else hasn't done so already.) The enemies in Mirror Jade are for the most part pushovers... Mirror Pandemonium is a test though. The first floor will be your building place for a long while. Once everyone has about ~3500 HP and 300+ MP, you can proceed to floor 2. (There's a HUGE spike in difficulty between the two floors, my 5000 HP 180 defense (that's second highest) monster is on floor 2.

Tips for survival in Rebirth:
1) Make sure at least 2 people know Holy and Flare (both are sold in Mirror Jade), Holy is in Mysidia.
2) If you are finding a high defense monster, don't hesitate to use magic. Square made it not suck (finally) so it is an offensive alternative.
3) Conserve MP or stock up on Ethers. I haven't seen anyone with Osmose yet.
4) Don't let me turn you away from this. :p

Spoiler:
1. Blink lvl 6 (not 16) + 2 shields + decent evade = untouchable. So keep everyone except one person in the back row, and have some people use sap/osmose to get rid of the opponent's mps, well after that they're toasted. Of course only do this to boss-type monsters. Flare and ultima (lvl 4+) is actually useful in this case to attack, or even items, if you're lucky enough to find a holy lance, use it!
2. you'll find "ultimate" gear for each of your person which boosts your stats by a lot, if you remember where you find the genji gear from pandaemonium, but do note that they're heavily guarded. Oh and go to the old masamune place last, that guy's crazy.
3. in the upper floors chests are genji gears, get them (if you can), don't hesitate to get back to town when your ether stocks are running low.
4. nothing is immune to flare/ultima, no flare? get it from that guy behind the waterfall in mirror jade.
5. minwu comes with a decent level sap, and nobody has more than 999 mp in the game.
6. teleport is useless. Warp saves lives (in terms of getting out of dungeons)