Gaesatae can be beaten if you pin them against a phalanx and then nail their unshielded flanks with missile fire or cavalry hits repeatedly (i.e. after hitting, pull back and reform). It tires your cav out, but if you keep it up, they break.

Anything other than phalanxes and they'll carve your boys up, but the long spears keep them out of hand-to-hand range usually.

Also, yeah, the Gaesatae are drugged up. Although they were not on it, PCP can do incredible things to a regular human being, making them almost super-human for a short time. I knew a guy who kicked a police-car door off, jumped out of the speeding vehicle, breaking his ribs and a leg, and ran five miles home still handcuffed, all on PCP.

In comparison, the Gaesatae don't seem so bad.