I have tried this several times and it works fairly well and is also quite reliable. However, it does take a while (up to 3/4 minutes) to actually get units into the game, so you'll have to be on your toes.

Build Order
Tunnel
Spider Pit
Tunnel
Upgrade - level 2
Tunnel
Tunnel
Spider Riders
Tunnel
Spider Riders

Optional Build Order (After previous one)
Fissure
Upgrade - level 2
Tunnel
Upgrade - level 3
Mountain Giant
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So the reason this works well, is because Spider Riders are one of the two types of cavalry that can switch weapons between spears and bows.

Because of this, they can mow down any enemy infantry or archers and take out from afar with their bows, any opposing spearmen or pikemen. They are also pretty decent cavalry with health being 600. This means that enemy cavalry shouldn't be too difficult to take out, unless playing against Men or Mordor, as Rohirrim are better armoured and Mumakil can simply stampede through them.

Spider Riders are also fairly quick to destroy enemy buildings, however, with the support of Mountain Giants (the optional build order), it should be much faster to dispatch enemies.

Additional Info
Spider Rider stats
CP: 60
Health: 600
Cost: 600
No. in single horde: 6