The difficulty spikes are rough for Rebirth 1 X(. At least on four occasions I got killed just from being either slightly under-leveled or from stupid battle conditions screwing you over (looking at you SP/Item/EXE Move blocker bot things X(). But, my advice from my 10 or so hours I played the game, I did the following and it made things a bit easier to tackle:
-Buy the DLC characters and level them up alongside the main cast
-Fight as many foes you can; you get lots of experience points and fill up the EXE Meter
-Do NOT use the EXE Meter unless you are in a main boss fight and give your party member who are going to use an EXE attack a Attack Buff; this from my time with the game, utterly trashes bosses in just a few turns
!
-Fight the 'extra' monsters you encounter after playing through the dungeon they are in; you get a lot of EXP points and also can complete very helpful quests
-Talk to everyone on the hub map, use the Disk system for really helpful buffs for the characters, make item/weapon/dungeon plans whenever you get the chance, and always have a high stock of items.
-Get the free item DLC's; they give you easy access to very pricey and rare items
-Always keep an eye on your party's levels and keep a sharp eye on your menus; you might get a new command to use or a new partner/party member and forget about it.
Edit: Also, from the footage of Rebirth 2, the main cast of CPU's become party members latter in the game (don't know how much latter, but it should be after you convince the CPU sisters in Lowee to help you out or something). So, they do come back into the picture latter in the game
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