Hey, guys. I just wanted to share my strategy for getting rid of those pesky spiders in early game. The images I have taken are from a newly started Silvan Elves campaign and every battle you see was my first attempt at them.
First off, we are at turn 2. My spy has located the first spiders in West Mirkwood and Legolas prepares to attack with the starting forces.
Under the deployment phase, I found the hill to my right to be the best battle ground. My sentinel unit deploys their stakes as far down as they can and Legolas takes position right behind them in loose formation. Further up the hill, my light and heavy archers stand and the sentinels join them after the battle has started. The enemy won't attack at once, so I send Legolas out to harass them. Around half way towards them, the enemy starts to march toward me and Legolas rushes back behind the stakes.
I don't know if it is the stakes or the massive archer fire, but this spidergroup always halts right before they are about to reach Legolas. If I kill the captain early, they rout before reaching Legolas. If not, I may take 20-40 casualties in the bodyguard unit. Here's the victory-screen for my first battle, not a single casualty.

In the next battle, we are at turn 13. Legolas has just returned from capturing Rhovanost and is reinforced by 2 wardens from Thranduil's Halls and the starting sentinel unit from Caras Galadhon. My spy has marked out the spidergroups in East Mirkwood and Legolas attacks.
The wardens take the front line this time (in loose formation), while Legolas stands right behind to stop the few spiders that will run through. As always, my heavy archers stand at a safe distance.
The end result for the second battle is 6 casualties in the bodyguard unit and 70 in the 2 warden units.

And lastly, at turn 15 I take on the last spider group in Central Mirkwood.
I followed the same battle setup and here's the victory-screen. Legolas suffered 3 casualties and the wardens 55.
Summing up, I took out 5 units of spiders and suffered minor casualties in Legolas' bodyguard unit and lost 125 men in the 2 warden units.