You fight Scorpioch at the end of Chapter 3 when you first reach the desert.
You need to destroy his 3 armoured parts. Both claws and his tail. Claws are easy enough but you'll have to jump for the tail.
Watch your health as your poisoned and it slowly reduces. The baby scorpions can be 1-hit soul seized for quick mana.
If you miss this then post game you can do a Challenge Quest called Gladiator I. This is against Scorpioch and some Fire Sirens, much tougher battle but can be gotten here aswell.