Well, purging was actually erratta'd down to only 6 inches, but thats still the best way to go. The lich's breath of corruption spell also works, and his high focus means you'll hit on average rolls. Skarre's has flys kiss and blood rain spells, and Dhenegra has venom, scourge, and dark seduction (your opponents will learn to loathe that spell when you start taking over guys with multiple attacks). The new bane knights can also make short work of them.
The thing is, most khador players keep their IFPs in tight formation to get the extra armor, so 1 bile thrall moving up and purging can catch nearly all of them, and if it doesn't, then purge another one. And if the rest of your opponents' army kills them, well so what. Your 41 points for a minimum unit just killed 100 or more, a great exchange by any measure.