It depends on the machine that needs to die...
Empire and Dwarves lack the ability to effectively move and fire.
As such, terradons work really well.
Elves (high/dark) can turn and shoot, and -2 to hit still results in terradons taking too many hits. A unit of ~12 shooters is enough to force a panic test. That's bad news for the LD5 flanking terradons.
Chameleons are good, the extra -1 to hit is nice, but you still have problems with BS based shooting. Being T2, those crappy S3 shots still wound too often.
My opponents have been castling up warmachines to protect against the terradons and chameleons, which leaves them very vulnerable to comet (go Tetto Eko!). I raked a gunline with a big tetto comet (came down bottom half of turn 2, with 4 markers, dishing out S8 hits; killed 2 cannons, a hellblaster and the engineer).
Speaking of Tetto, I've used his vanguard ability to rush up skink cohorts and salamanders. Both do just fine at killing warmachines.
I think the Spirit Leech through the skink is awesome. Warmachines often end up outside of the generals LD bubble and not only should you kill the machine, but you should pick up a power die in the process. Death Magic gives you 3 direct damage spells, 2 of which are very good (hint, warmachines have no strength, so 2D6 - strength = 2D6). The high toughness makes the 3rd death spell less attractive.
I also like skink chief with cloak of feathers. Light armor, enchanted shield and scaly skin gives him 3+ armor. He can charge out of a skink unit turn 2 typically. Init 6 and 3 S4 attacks is decent. At 82 points, he's pretty cheap.
-Matt