I'm interested in ways of providing alternate avenues for the player to succeed - potentially easing mechanical difficulty beyond just a difficulty setting.

One idea I really liked and don't see in most other games is semi-non-linear progression with powerful specific items elsewhere like Gauntlet games used to offer.

So say the Chimera is way too hard for you at the moment, then you can move on and progress elsewhere at the expense of his rather great rewards until you eventually find the Scimitar which instantly cuts off one of his heads at the start of the battle, both reducing his health and removing an entire section of attacks. The Sword comes a bit later in a different world and isn't really easy to get even then, so its not a freebie but its an option.

This gives the players two distinct offerings to dealing with the difficulty. The skillful way to just kill him and get his loot/experience early on when you need it most, or the clever way where you just move on and return with a better chance.

You could even not make it so simple as just an instant attack, but something like a unique weapon that mechanically changes how you deal with it. Something like Demon's Souls Storm Ruler or DS3's giant slayer sword that lets you deal with the boss there in a completely different way, but actually putting it elsewhere instead of in the boss room and making the boss slightly more reasonable to kill without it.

Essentially treating stage challenges as combat puzzles, which the player could "solve" through secondary means.

I think Ghost of Tsushima might have done this one of the better ways I've seen. In that you could spend a lot of slow time using stealth to reduce the enemy numbers or you can just walk right into the room and hit the Stand Off button to play a risky game of proving your skill to instantly terrify a bunch of them out of wanting to fight.

And then once the combat begins you have all your different stances which provide both direct counters to certain enemies if you prefer, or simply an ability to adapt to what might be needed to deal with the encounter before you. Whether its playing defensively to angle for instant kills, running fast to get to the archers before dealing with the main group, or ruthless aggression to clear them out as fast as possible. And this is without getting into the options of what gear you put on to enhance which parts of your kit, thereby leaning into those particular moves over others.

It also offers what I think is the only "acceptable" difficulty level upgrade over normal gameplay. Which is the "swords work like real swords now" type, aka both the enemies and you become incredibly lethal. So while its harder, its also not just numerical but rewarding.