Survival Sword

There are a few things about this scenario that jump out at me.

In the real world, suiting up in diamond armor would be a stupid idea. As pointed out by the numerous other commentators, diamond, while very hard, is not exactly durable; mundane diamond armor would shatter easily, making it quite probably worse than useless. However... we are talking about something granted to your protagonist by a god here; you could easily set it up as, say, magically forged, perhaps giving it properties similar to 'mithril' from The Lord of The Rings, for example. I admit that personally, if this really is a god of wealth, I'd be more inclined to imagine something like a pure platinum armor set encrusted with precious gems (platinum is probably not great armor material either, but the real armor in this scenario is the god's magic, not the mundane matter it swims through.)

I haven't checked out which of these options (diamond vs. platinum) would be more valuable in our world, I admit - but value isn't something intrinsic to the valuable thing; it only ever exists in the context of a society, a culture, an economy. Diamonds are expensive because they're rare, and because enough human beings place a sufficiently high value on them to drive up the demand (partly also because they're rare and, of course, shiny) - not because they are diamond. In your world, maybe the economy works differently than our modern one (though I'll take it as a given that gems, precious metals, etc., are still valuable.) Which segues nicely into another point...

As a god of wealthy people, it would be incredibly counter-productive to try and kill off the poor. Why? Well, without the lower classes to take care of the "mundane" work for them, the rich would be forced into manual labor themselves. If you know something about history, you'll know that before mechanization - before the invention of such marvels of technological cleverness and infrastructure as the washing machine and industrial-scale farming - everything that needed to be done had to be done by hand. The only reason the rich and well-off can be rich and well-off, as we commonly understand it, is that there are whole strata of people below them doing things like the manual labor of farming (without which there would be nothing to eat) and building (without which there would be no shelter), weaving, tailoring, cooking, mucking out stables... the list goes on. As a god who wants to preserve and increase wealth in such a world, I'd probably be more interested in suppressing rebellion and making sure the lower classes don't get any ideas in their heads, speaking for myself.

That doesn't mean that The Expensive One won't do what the premise states! It just means that I think their/her/its motivations are probably rather different than it might seem at first glance: systematically savaging the poor and working people will create a great deal of economic instability and (if you can keep it up) might quite conceivably cause the society to collapse.

Getting back to the point of this question, finally... I don't think that simple armor (whether mundane/relatively ineffective/impossible diamond or something that's effectively mithril) and a weapon will guarantee you victory over the King's forces all by themselves. Even without supernatural aid of their own (they have the whole resources of the kingdom, which may quite possibly include mages or the priests of other gods) they probably could, if you tried to take them on head-to-head, eventually trap you (with, say, weighted nets), chain you somehow or other and seal you in a deep crypt to live out the rest of your lifespan in darkness (see: 'doesn't feel hunger or thirst.') They have numerical superiority, after all, and taking them on in an open area where you can be surrounded ... might not be the best idea, especially if all you have is relatively mundane weapons, armor, and superhuman durability.

But that doesn't necessarily mean all is lost: As a canny protagonist who wants to keep causing economic chaos, pleasing her god or maybe just sacrificing the fingers of the unfortunate, I feel like I'd try a few other approaches...

First off, I'd probably try to evade the army or lure them into a trap of some sort: perhaps draw them into a narrow, twisting canyon or even a tunnel system where I can attack from the shadows and pick them off one by one, or at least two or three at a time. One big advantage shows up here: they'll get tired, scared, thirsty, and so on, and I won't.

If this wasn't feasible, I might want to become a sniper: find good vantage points from above the enemy force and fire down onto them, preferrably causing as much chaos as possible with each shot and moving off quickly to avoid being targeted in return - improvised explosives, fire, or maybe even some kind of cursed payload would probably be good ideas here.

Finally, assuming I've done a good job slaying and sacrificing peasants and such in the past, I could always pray for assistance from the god I follow. I have no idea what The Expensive One would be able to or willing to do, of course - if it would even do anything...