I feel like I might want to try playing the first, but it was so painful when I first fought Youmu. She just tore me apart as the wake up call boss.
But that's exactly what makes it fun. She's the first 'serious' boss fight in the game where switching strategy really counts. Especially since it's her ghost half with its HP-shredding poison effect that's the real danger. Youmu's slashes are dangerous too, but at least they're manageable with buffs. If you're having trouble with a boss, I'd suggest always taking the time to fully explore the level before touching any boss tiles. That usually helps make sure you're levelled enough. Also investing points wisely helps. Affinities are fantastic for mid-late game as so many attacks are elemental, and having a flat damage reduction really helps even on characters with poor DEF/MND. If you're taking only half or a third of the damage from an elemental attack, even someone flimsy like Chen could potentially take a hit and survive to switch out.
Spending levels in stats like HP and DEF generally helps make you more powerful in the earlygame because of how scaling works, whereas massively boosted affinities really shine later on where damage scaling gets huge, and gigantic damage reductions can be had from massive affinity boosting. I'm currently running a game where ALL levelup points are dumped into affinities for all characters just to see what it'll be like.
My Youmu fight ended up with Remilia at 10 HP as the only character alive in my party alive finishing Youmu off with one final Spear of Gungnir she barely had the SP for. I think I tried hard to get speed debuffs to stick, but it ended up being better just to have Cirno and Reimu desperately try to PAR her in between getting picked off...
The boss of Floor 4 is also really fun, especially if you don't have someone in your party who can inflict Poison.
I'd say definitely go back and play the first game. It's well worth it, and the pacing is very different from the sequel. There isn't too much that's patently unfair or bullshit and the stuff that does catch you off guard the developer specifically put in to mess with you (Floor 5 boss, hello). The first really, REALLY nasty boss I'd say is Floor 7. The fight is actually pretty predictable but even when you know exactly what's coming, the sheer damage output and speed can wipe your frontline in a few turns. You need to spend a fair chunk of time on the floor grinding levels (which is recommended anyways as there's great drops on 7).
