The Drifter is a charred rotinaj with no limbs that floats off the ground and hovers towards it's prey. It spits congealed blood at it's target obscuring there vision and hitting 3hp. The Drifter can spit twice in a second meaning that in large groups they can be lethal. It costs the Zombie Master 25 resources to produce making it a rather cost effective unit.
BASIC: Drifters are dangerous. That's the first thing you have to understand. Drifters spit blood at 1 metter of the survivors, which means that walking pass them is not a possibility. When a survivor is hit by one of these, their screen will shake making them lose their orientation, and obviously, confusing them. Drifter "spam" is much more expensive than shambler "spam", which means that you shouldn't really do it. Drifters are like butter, they combine with everything boosting all of other zombie type's strenght and effectiveness. Drifters alone will only get killed except if they're massively spawned (and even then, barrels or molotovs would wipe them out easily). So use drifters in combination with hulks, banshees and specially shamblers. There's really nothing more to say about these guys, except that you don' t have to ignore them. Don't think they're a most expensive version of shamblers, because they aren't. Use them, play with them, but do it well.
ADVANCED: If you're looking to improve your skills with these guys, you won't find many ways to do it. It's almost everything about knowing when to spawn them, and how many. Drifter walls are a possibility though. But be carefull not to build an expensive drifter wall if there are barrels near there, or molotovs, or you could see your resources burning as fast as lumber. Using drifters is much more risky than using shamblers, because they cost thrice. Still, they can be great if used well (which is not hard).
MASTER: For master using of drifters, place them near cliffs and precipices, so they can make survivors fall down if they decide to try rushing through. Also, spawn drifters when there are several humans holding barrels, as their range attack has more chances to blow them up, blowing away the players aswell. Finally, notice that drifters are more effective when used in dark and narrow places, as they're absolutely black and cannot easily seen.
Aim for the red area on their body. Whenever you see a drifter which is blocking the only way you can use to keep moving, kill it immediately, or you will have problems. Do never ever get in close range with these guys, always avoid approaching to them. If you fall in a drifter ambush, try jumping around and finding a clear path to get the hell out of them. If you face a drifter wall, pop a barrel near it or burn it with a molotov. If you don't have any of these, just shoot them as fast as you can. If you can run pass a drifter and ignore it, do so (they're much more strong than shamblers). If a drifter is coming after you and you have a barrel, throw it immediately or you're assuming the risk of it blowing up.