Topics covered in sections below:
Would a typical Tolkienesque / D&D-style elf often lose the fight? As a trope such fae-folk avoid armour, heavy weapons (especially pole-arms), and seem to go out on foot. An example at Helm's Deep - faced with heavy infantry after the wall fell, the elves were wiped out in seconds. As troops, hand-to-hand combat between archers and heavy infantry seems like an unfair comparison, so let us look at orcs for a second.
Orcs have everything those fae-cousins lack and tend to live and breathe fighting-killing over culture and sophistication.. Heavy armour, check. Heavy weapons, double check... and they are usually carrying spare weapons to boot. Orcs also use cover, siege weapons and proper combat tactics. Consider orc fortifications and compare that to any elven towns you might find on deviant art. Can you find a single elven town even with walls? Where are the ballistae?
Overall, elven settlements are looking a bit like a meat co-op for orcs.
Say you crank this to 11 when you add orcs + Tanarukk (especially if guided directly by demonic lord Baphomet). Even those orcs without their six-pack of gods for guidance: let us assume they are overtaken by a wizard-necromancer complete with undead grinder-zerg attaché. Bonus D&D plot: a drow-wizard pretends to be an orc by night (whilst leading orcs) and a high elf visitor by day ('light skin & sunglasses'). Scouts out the town before the raids (disguised as a 'day elf'), plans out a strategy whilst putting on their orc-disgust and prepares the orcs for the nighttime battles. Why can't such a wizard slaughter everyone?
Following this logic: you might assume entire cities of elves have been slaughtered for many generations. The few remaining survivors would have to get resourceful. The ones that wanted revenge would have to double down on how much damage they can do with few allies. This line of thinking is what led to this guide: the Mist elven are a magical evolution of the elven archetype - but deeply invested in revenge.
How do elves win without heavy armour & siege-walls - especially against orcs that are both hardcore and metal The trick? Destroy the enemy at a distance - and don't let them strike back. Better still, convince the enemy to destroy themselves. But how is this all done?
Two things are key: damaging the enemy from a safe distance and getting the enemy to kill themselves. Fortunately for the Mist elves, most of their foes are chaotic and evil. This means they lack both discipline and reserve. In short, chaotic evil creatures rarely trust one another and tend to seek for violent solutions to any problem. This is ideal when your enemy seeks to fight but only have one another close by.
These elves focus on their mastery over water: specifically mists and fog. This means they can produce, (re)shape and 'see' (divination) water, specifically mist. For racial spells they will need Shape Water, Fog Cloud and Levitation. Standard gear & skill-racial abilities include cloud-blending clothing plus exceptional cloud stealth. Their ability increase would need to be in charisma (they would be excellent sorcerers, warlock-witches, (vengeance) paladins and even bards that specialize in mind-control.
In groups of more than 20 Mist elves can control weather to change it to 'fog'. They would also have a number of fog-related spells such as Cloudkill and poison such as Malice to create blindness - this plus Confusion is an excellent tactic.
Levitating above the enemy and dropping spears (especially ones stolen from orcs) at 200' works - but too slowly. This is a great way to take out single targets, especially with Purple Worm poison on them. The spear does a d6, the fall does 20d6 and the poison does 12d6 (save for half). With added dexterity (+2 to +5 or so?) and you have a non-magical attack for 120 damage or so. Three of those dropped by surprise onto a demon lord means an instant win - and by using a stolen spear it would look like an attack from a rival orc clan. If you can find someone within 60' of the landing-target, you could even add Catapult to increase chances of a hit.
Spreading diseases is also key for a proper terror-oriented attack. To have your enemy shrieking with mad laughter from Cackle Fever works really well to break the discipline and morale of troops. This disease is often spread through careful additions to local alcoholic beverages - or by adding 'gift' kegs that appear to be identical to their own supplies. The disease need not effect everyone - so long as enough of the enemy are shrieking with insane laughter in the mist to ensure no one trusts anyone.
These are all tactics that assume a few hundred Mistelven seek to irradiate tens of thousands of troops. This is very difficult task to accomplish without enlisting the ironic help of one's enemy (and their collective urge to kill stuff, especially one another).
Shape Water (cantrip) - able to take a 5' cube of water and translate that into a thousand times more mist. Also: instant ice rinks in fog are surprisingly powerful. One can also shapes / messages the mist and communicate with one another silently. This also allows one to change the opacity of any water - though a 5' cube of transparency isn't much. You may want to give them a racial cantrip that allows them to see through a familiar's blindsight up to 500'.
Fog Cloud (1st lvl): 20' radius sphere is small, but if a few hundred Mist elven cast it, this could easily turn the tide. One could rule that they can gather as a ritual to cast this: each five working together add a level of slot. Casting this multiple times at 100' radius would work well. Faerie Fire is a wonderful spell but it only works if others can see the target(s) - this may or may not include blindsight. Unseen Servant would be surprisingly useful in the fog, causing plenty of confusion and frustration. Tasha's Laughter may also work well if the enemy is unsure who is laughing at them / cause riots within the fog confusion. Hex and Hunter's Mark do NOT allow one to track targets, so the are fairly useless.
Cloud of Daggers & CloudKill are both fairly useful in deep fog: the target has no idea where to run. Blindness and Deafness magic is useless as it is far easier to simply mimic both friend and foe sounds in thick fog and let the enemy react as they will.
Magic Mouth is surprisingly interesting, giving the endless whispering of the landscape in twigs and rocks. That said, there is no role-playing schematic for the terror caused by whispering voices that one cannot find nor the chaos that would ensue if someone believed they were surrounded by ghosts. The up-side of this spell: it is infinitely reusable if one words it correctly and can retrieve the object-of-mouth later on.