Yup, all units aren’t balanced…
I would go with Sphinx BTW, unless you have a good troop/weapon using purple mana (then it would be green spider).
However, I think you got the wrong idea of how to use properly the Black Beast.
You use it to “save” the attack power of other troops when they are about to die.
I believe, you should put the Black Beast in frontline, and just before dying, devour another unit. You will heal your Black Beast to full + assimilate stats from the devoured troop. It’s clearly not very effective, but as you assimilate more units, you will replenish more life and accumulate attack damage.
It’s not even good in my opinion, but not as bad as you seem to think