First of all, Mericarols and purple epsilons one hit kill and they're in the game since gamecube.
Back when I played PSO as a kid on my friend's gamecube, we got oneshot by the first hildebear encounter in normal. What did we do? We devised strategies to deal with it regardless, and that's some of the most fun I ever had playing PSO.
But when you have demons, mag invincibility, guaranteed freezes, 4 way PB and J/Z at your disposal, the fight is already favored heavily towards players. Especially in terms of pure power. Several bosses get obliterated by dark flows and final egg blasters before they can even attack.
The truth is, a team of geared Lv 200 players is much more powerful than the content currently available (events excluded) and any content without at least some instant death mechanics is just too easy to be called endgame.
Another problem is the fact that enemies can only have a maximum of 32767 hp. But even if we were to increase that somehow, demons would make light work of any HP increase. We do have plans to address this issue in the future though.
An argument could be made for tanking, healing and dealing damage, but with the utility we have available there's always going to be a better way to kill enemies more efficiently. Plus, enemy positions and targets aren't synced in PSO, so an enemy may be attacking the tank on one client, but going after the healer on their screen.
All this makes balancing new content a difficult task. But I believe we did a good job with the right combination of power and instant death. And if it does turn out to be unreasonably hard, we can always nerf some parts of it.
Lastly I want to mention that, without saying too much, this harder content will benefit everyone in some way, even players who aren't geared enough or aren't ready yet to participate directly.