Scorpions are even easier to take out than ballista elephants, they’re clunkier and slower, and they have minimum range.
An onager (the regular one, not the siege version) shot decimates scorpions, ballista elephants need more.
20 onagers win vs 40 heavy scorpions easily, provided they have siege engineers if the enemy is Khmer or with a Khmer ally.
Siege Onagers completely destroy scorpions.