Generally, this guide is about powerful items, more specifically, about items that add damage to your troops, either by a flat amount (usually +1) or by percentages. Also note that even though you may be lucky enough to find the best item combination for a group of units, you may not use them efficiently because those units aren't for sale anywhere.

Crown of Blackthorne

+100% physical damage to all Thorns (3 units qualify)

Thorn Hunters and Warriors are level 1 neutrals with only basic attacks and seeding abilities. Normally, those troops are not among the mightiest, but with double damage, they are at least as effective as other armies, if not even stronger. Additionally, you can have the Royal Thorn that summons Thorn minions every 3rd round and shoot the enemy in cooldown phases and you can have the Dryads, which boosts morale of Thorns by 1 and is also able to summon Thorns.

As a general unit setup I would recommend:
Dryads - For Thorn summoning and CC abilities with sleep
Fauns - For ranged attack, CC sleep and Dryads Initiative
Royal Thorn - For Thorn summoning and ranged damage
Rune Mages - To make Royal Thorn / Dryad Phantoms and restore lost runes with Runic word
Ancient Ents - As sort of tank to catch the fire of enemies

Helpful spells would be:
Runic word - To enable your rune mage to keep phantomizing without need to use his skill
Target - To make your enemies shoot your tank
Weakness - To make your enemies deal less damage
Phantom - To summon additional Royal Thorns / Dryads

As additional items I'd recommend damage boosters like Gloves of Death, Icy Spear, Whip of Pain and Assassin's Dagger as well as battlefield control like the Voodoo Doll, Hunters Boots and all fire resistance boosting items.

1 - 3 Thorn Warrior Base Damage
2 - 4 1 - 1 Added fire damage from Gloves of Death
3 - 5 1 - 1 Added cold/fire/poison dmg from Icy Spear / Whip of Pain / Assassins's Dagger
5 - 11 2 - 6 Added physical damage from Crown of Blackthorne

1 : 1 Leadership 8 : 8 Average Damage

Jade Belt increases all Spiders damage by 50% and increases their Initiative by 1.
S'Karrash Shield increases attack and defence by 9, Initiative and speed by 1 and Spiders always deal critical strikes. (5 units qualify)

Spiders come with a native high damage, speed and initiative. Coupled with those two items, the spiders are just so much more powerful. The Cave spider is almost always available to buy or hatch from eggs. The other spiders (Fire Spider, Spider and Ice Spider) are somewhat rare, but the Ice Spider is available after doing the quest for the guy on the island in Arlania. There would be also the undead spider, but those are not available to the player unfortunately. As 5th troop you should take something that can efficiently deal with Ancient vampires like Inquisitors, Priests and Demonologists.
If you use the spell Haste and have other items boosting their speed (hunters boots), the Spiders are capable of reaching the enemy lines on the first turn, mostly killing them off right away.
As additional items I'd suggest damage boosters like Jolly Roger's Cutlass, Icy Spear, Whip of Pain and Assassin's Dagger as well as battlefield control like the Voodoo Doll and Hunters Boots.
The downside of this tactic is the rare occasion of getting all 4 types of spiders and the inability to deal with Ancient Vampires.

On a side note: If you are playing as warrior, you have that skill that increases critical damage by up to 50%... So as a small calculation:

2 - 4 Cave Spider native damage
3 - 6 Bonus from Jade Belt: 50%
9 - 9 Bonus from critical: 50% max dmg
10.8 - 10.8 Bonus from Jolly Roger's Cutlass: 20%
15.3 - 15.3 Bonus from Warrior Skill: 50%

1 : 1.09 Leadership 14 : 15.3 Average Damage

That means you do 5 times the average base damage.
You can get a further damage booster with Dragon Slayer or Power of the Night/Light spells