Couple people foresaw what in hindsight turned out to be the burning of the boats and the crossing of the Helcaraxe. Several people have vivid prophetic dreams of how they die. (Those have always been shareable with almost anyone). 

How well did the visions of the crossing match what happened? That might be a pretty definitive answer either way on whether this works.

There are options for dealing with visions of deaths. People who think it might be soon can move to a safer city for a while. But if it's shareable they could have done that even without a human involved.
Amber asks the King about special consideration for awakening for anyone who knows how they'd die. They're not likely to die in a fire if they're a halfway prepared elementalist, or if they see themselves being stabbed in battle they can expect not to be in many sword-based battles...it's not a guarantee but it's a pretty good shot at avoiding most specific risks. (Hopefully none of these people would be terrible practitioners.)

Most people dreamed the crossing as more unpleasant. The deaths mostly aren't soon, and mostly are in fact in sword-based battle, or generic 'consumed by sudden fire'.

Regular battles are going to happen no matter what, but if someone's part in them looks more like artillery from a distance or directing summoned bogeymen they can at least be pretty sure they're off script.

Fire's weirder. A lot of people burning suddenly is oddly specific. No one can know that the two groups are getting premonitions in anything like the same proportions, but if so they should be amassing elementalists to fight an army-size fire instead of doing it on an individual scale.

...yeah, that's a good idea. At least they know where to find lots of temperature elementals.

Possibly wasted effort. The Enemy might just try killing a substantial fraction of the army with some other disaster instead. But they can't not prepare for this one, so.

And the Enemy wouldn't know his original method won't work.

It's a large-scale project and they'd have to keep it secret for a long time on an Elf scale. Is the Enemy's Vala magic slow enough that he'd have to have already started building up for the fire plan?

Well, they have no idea when it happens, so that's hard to answer.

They can try for a bound on that. Upper or lower, depending. If the war absent magic involved founding more kingdoms it must have ended up looking stable-ish with enough safety margin that they don’t need everyone here. The King might have a pretty good guess at who would be leading those and more importantly when under those circumstances they’d leave. If the people with premonitions about fire nearly all think they’d stay at the front, the Enemy does his thing after whenever that is; if they’re as likely as anyone else to predict they’d settle elsewhere, the Enemy moves faster.

It might not be a useful bound, but it also might be.

They are generally disinclined to predict they'd settle in some civilian city far south of here, but it's not a universal and there's nothing else noticeably different about people who expect they'll die in a fire. It could also be that everyone dies in a fire but the ones who got prophecies about it are the ones whose behavior wasn't affected.

Probably not that one; in the other timeline some subset of them so spread out. Even the Enemy shouldn't be able to throw around that much fire. Hopefully.

So that suggests it's coming up soon, by Vala standards at least. A smallish fraction of the sixty Years.

Weakly suggests that, at least. Ice elementals?

Ice elementals. While steering clear of the area that used to be Irissë's demesne; that's pretty thoroughly property of that one spirit.

Amber doesn't go with that group. She's more useful at the stolen library, translating and checking other people's translations and debating whether to open the diabolic texts.

They meet no disasters, and return with abundant ice elementals. They're prepared for one potential disaster, at least.

One down, all possible disasters minus one to go.

They're still awakening increasing numbers of people. Eventually maybe magic will just have to get used to not being secret at all. In the meantime the practitioners split between distracting the Enemy and learning the (relatively) promising avenues for hurting him safely.

They have enough people at this point to do enemy-distracting continually.

The Fëanorians definitely know how that trick works by now. They speak English.

Doesn't mean they can drop the complicated-looking decoy machinery; the Enemy doesn't. The fraction of his attention they're taking gradually ticks upward, well past the level that drove him out of Angband last time...

He does not leave Angband. He does not noticeably retaliate, either.

It's better than the alternative. (Unless he found a way to block it while making it look like it works, and is just letting them waste their own time. Which would be hilarious if it weren't also the exact opposite of that.) His other projects have to be suffering for it.

One assumes. They can't get feedback more detailed than the location, obviously. The Elves are disinclined to escalate when the Enemy is currently not doing anything horrible, and when the war is supposed to drag out for sixty Years.

It drags out for sixty Years and they lose. Not that Amber's pushing the issue right now; they don't have much to escalate with yet. Since no one is touching diabolism, there aren't many methods with much of a chance of affecting a Vala.

One is summoning. There's a variety of places Others can be summoned from, of which the promising one is the Abyss. It's the place forgotten people and objects go when they do what gets described as fading away or falling between the cracks. The Abyss has a claim on all lost things, and when it gets them it grinds them down both physically and mentally. People are forced to trade away bits of what makes them themselves in exchange for survival, and the Abyss fills the gaps. A few manage to escape and find a new role as bogeymen killing others or dragging them down to experience the same.
Mostly bogeymen are useful soldiers when controlled but nowhere near the power level to take on a Vala. But humans have forgotten many things over the years—it might not be completely impossible to summon a god. The obstacles are that anything down there has by definition been forgotten, so they aren't going to find any names of things that haven't been rediscovered, and that they don't (yet) have any bindings general-purpose enough to use and strong enough to trust.

The other is enchantment. It can get much more solid and direct than it is by default, binding people almost as much as an Elf's oath can. They don't have instructions for how to do this, but they do have lesser offensive uses. Severing connections permanently, making an arbitrary rock the most important thing in the world to the victim, a lot of fun unethical stuff. They can try to extrapolate to the degree of control it'd take to lock down a Vala permanently after getting the upper hand. Marginally less unachievably, they might be able to make a captured servant of the Enemy incapable of connecting "Melkor" with "the ruler of Angband" and get them to switch sides.

The latter might even be safe to try with orcs.

They wouldn't gain much from doing it to an orc, but as proof of concept definitely.

Orcs are acquired.