It's rather easy. I completed the trainer dungeon and the first four "real" dungeons and have yet to die. Maybe I'm playing extra carefully because I'm used to the SNES and DS games, but I have had relatively little difficulty so far. My ally got knocked down to 2 hit points in a boss fight once, but I had plenty of healing herbs by that point. And Recovery Staves, which heal for about 30 damage, are extremely common in the early game, although they have the drawback that you can't use one on yourself, you must hit an ally with it.
Hopefully the challenge will kick in soon, but the whole reason I gave up on Pokemon Rescue Team was that it was dreadfully boring in the early game. At least I know that building up a character isn't required up to this point; I have entered no dungeon more than the first time I entered it, and I haven't used the