The Daearn Line
A team of giant eagles fly overhead, masked by an Improved Invisibility spell. Each carries a veteran elven warrior, also invisible.
At a command, the flyers drop their riders, who plummet to earth at enormous speed, halted seconds before impact by a single-use feather fall effect. Landing at a strategic point between enemy units, each elf places a Daern’s Instant Fortress and speaks the command word, springs inside and closes the door. In mere seconds, a formidable strongpoint has appeared in the midst of the enemy. As yet, the elves are still invisible.
Inside, the elf opens a Bag of Holding and tips out 64 cubic feet of pre-animated skeletons with longbows.* These are sent to the arrow slits, where they begin massacring anything that gets into range.
An average human body is 66l; a skeleton is around half this, or 30l, equivalent to one cubic foot. Allowing for longbows and arrows, the Bag of Holding contains approximately 30 skeletons.
Certain of the Fortresses have permanent teleportation circles etched inside; these are not furnishings and so do not need to be removed for the Fortress to operate. Once the fortresses are activated, a mage uses a teleport scroll to travel to the Fortress with eight elven warriors, each armed with a wand of fireball or equivalent heavy artillery. As high elves, they qualify as spellcasters and can thus attune to even powerful wands regardless of magical training. The warriors take up station at the arrow slits and begin expending their wands, wreaking havoc on the enemy lines. In some cases, they instead have plant growth, move earth or other terrain-affecting wands, and are transported to crucial locations where the enemy can be heavily disrupted or separated, leaving them vulnerable.
As soon as serious danger threatens, the elves order the skeletons back into the bags. They either open the door, or climb to the top of the tower, whichever seems safer. Speaking the command word, they shrink the fortress back to its cube, pick it up, and invisibly escape. If necessary, they activate a single-use teleportation item to return safely to base. Wand squadrons instead activate a second teleport scroll to return to base, before the drop-elves retrieve the tower and escape.
Tides of the Earth
Sapper squadrons are composed of elves who know the mold earth cantrip. Since no combat training is required, civilian conscripts are often used, including prisoners in penal battalions, and trained to respond to particular commands. A squadron of 100 sappers can dig a 500'-wide trench within six seconds before retreating behind the military line.
In preparation for retreat, the sappers flow to either side of the elven line, and create impassible trenches to prevent encirclement and narrow the line of pursuit. When the retreat is sounded, casters lay down a barricade of thorns and ensnare to delay enemy pursuers. While archers provide covering fire, the sappers fall to the back of the line and rapidly excavate 5' trenches in succession while falling back. They can create one such trench every six seconds; depending on urgency, they may dig trenches 30' apart, or only 5' apart. The latter creates a serious obstacle to pursuit due to the effort involved in repeated long jumps or climbing, and essentially prevents cavalry from following because they cannot safely land on the small area nor gain the distance needed for repeated jumps.
A group of sappers are given Bags of Holding containing only air, and Eversmoking Bottles. They walk slowly under the battlefield, moving earth aside behind them and using the Bag of air to ensure they do not suffocate. They arrive underground amidst enemy lines or inside a defended fortification, where they carefully create a 5' deep thin hole to the surface and embed the Bottle underneath it, activate the Bottle, and walk away. Smoke pours to the surface, blinding the defenders while the elven army surges forward unseen.
Protected from arrow-fire by Eversmoking Bottles, 100 sappers approach a fortified hill or position and begin removing the earth from directly in front of them, walking steadily forwards. As they approach the steeper parts of the hill, earth from above falls down into their zone of effect and is removed in turn. The entire hill is steadily brushed away until the fortification collapses.
The Hungry Rain
Trolls are hunted (or even farmed) deep in elven territory. A finger is cut from the troll before the body is slain and burned to ash. The finger is placed in a mild acid solution, typically lemon juice, which is calibrated to prevent regeneration while not doing enough damage to destroy the finger.
Eagle-mounted warriors soar high above enemy lines, seeking a target: the baggage train, the siege weaponry, a fortified stronghold with a courtyard, a site where weapons and armour are manufactured, even an enemy town (war is a cruel business, and stopping them providing any food is one way to win). Circling overhead, they upend a Bag of Holding, and hundreds of small jars tumble to the ground, shattering on impact. As the lemon juice seeps away, the fingers within rapidly regenerate into adult trolls, which howl with rage and immediately seek to slaughter everything in sight.
Due to the sheer devastation caused by this tactic, and the lingering effects of stray trolls wandering through the territory, it is typically used only deep within enemy lines lest the elves fall victim to their own weapons.