The Vyper

The 'Edsel' of Eldar technology. Open-topped with an armour value of only 10, these skimmers are a smouldering heap of wraithbone waiting to happen. If you give them a useful weapon (Star Cannon or Bright Lance), a holofield and a spirit stone to try to keep them alive for more than one turn, then they cost a bundle (100 points). Almost as much as the exponentially more useful wave serpent. If you don't use options to protect them, a small squad of Tau Fire Warriors with Pulse Rifles can wipe out a squad of three with their eyes closed.

The only time I use them? Against Tyranids. Venom Cannons can ONLY cause glancing hits. With the holofield and spirit stones you can survive most (and even ignore some) glancing hits. Their speed and agility should keep them out of range of any other weapons. The other plus is that even basic Vyper weaponry (the Shuriken Cannon) is effective against most bugs. The strategy is even more effective if the Tyranid player has only a few Venom Cannons and the Ulthwe field a large number of tough (Wraithlord) and/or armoured (Falcons, Fire Prisms, War Walkers, Wave Serpents...) targets.

If you want to see a Hive Tyrant's head spin around like an insectoid exorcist, just give it a dozen horribly be-weaponed targets to shoot at.