I feel like we know nothing of the inner residents. We could assume that they're something akin to the Magus. Rank and soul evocations yielding unique, powerful options from each. Their civilization benefits (somehow) from the trapped Ring, drawing power or sustenance or some other benefit from its imprisonment. The Magus bore a magical dagger, so their fine equipment will be similarly enhanced. Even without his spells he was a superhuman combatant skilled in combat, so we can expect better-than human abilities.
The term 'outrider' implies but does not guarantee cavalry. Their mounts will certainly be of superior stock, but whether they're terrible beasts dangerous in their own right or optimized tireless fast ponies, I can't say. Further, they patrol. The first function of a patrol is not to defeat threats, but to identify them. It may be more important to have Gisena shut down magical means of communication, rather than aid us in the battle directly. Unless they have frickin radios or horns or whatever that doesn't need magic, ofc. Regardless, we need to consider this a fight with a time-limit, because they certainly have allies somewhere, even if they're not immediately running away to find them.
Diplomatically, we're a crazed sexy murder-hobo with an also-crazed also-sexy sorceress in tow. I don't think we're holding many cards. The greatest desire of these people seems to be isolation. They're not making any visible attempts to interact with the outside world besides killing the bits of it that intrude. We could perhaps offer our assistance in this matter, but participating in the wholesale slaughter of innocent hapless adventurers seems a bit much. Particularly given that we'd be actively plotting betrayal as soon as we're granted passage further inside. For another angle of deception, we could play the lamb trotting to slaughter, and double their supply of stupid trapped ring-bearers! Pretty awful idea though, since we'd immediately be carted off to someplace with even stronger opposition, and well-established means of capturing and exploiting supernatural jewelry.
Straight intimidation? Actual no-kidding seduction? There isn't much nuance to be had in these options. I got nothing.
In the actual combat, I expect mobile, canny combined-arms tactics featuring several distinct fighting styles designed to leverage several distinct soul-evocations. I don't think they'll have a weak link, but it's likely there will be a path of least resistance. Hopefully we can spot and attack that point before they can take advantage of our focus, and remove an element of the opposition. They're a match for us as a unit, not as individuals, so hopefully a casualty would tilt things in our favor.
Perhaps intimidation would be more viable after a death, and such methods are extremely, straight-forward, but I wouldn't hold my breath. More likely they'll attempt to disengage or fight to the bitter end, rather than take a break to listen to the one-armed man sling insults.
Actually fighting mounted opponents as a dude on the ground sucks. Every attempt should be made to seek more favorable terrain. Fortifications, forests, marsh, probably not marsh (it sucks), anything that provides impediment to rapid hooved motion. If they got clawstriders and ride charms then I guess we just die.
Exhaustion seems to be a big advantage for us, with the Ring in power. These guys don't immediately demonstrate Ruin, or the power to swat us down immediately, so a protracted fight might be to our advantage. Drag things out by repeatedly disengaging, use the ring to tire them faster while our blood remains fresh, things like that. Again, being the guy who's walking isn't great for this.
It's obvious enough to go unstated (I hope) but kill the mounts if you can, baka.