Justice is a fan-made global mod.

3) Hierarchy might suggest that, but visually, witches can't be stronger than hydras.

Physical strength is the least important factor here. 5th-tier hydras are powerful but dull and slow, while witches are intelligent, calculating, and stealthy, plus they are good at casting spells. They are useful and more important than strong, but dumb, muscle in any case. I would say the same about griffins/monks, but the role of griffins in imperial troops is much more important, and monks are buffers like shamans in orcish armies, and their 4th level can be justified.