I do not know if anyone has suggested this before and I can't be arsed looking that up because there has been tons of threads about nerfing Acheron but here's my simple go at it.
Bring the combo damage back to the earlier values, keep the charge's current damage. Why so?
Let's treat charges and combos as separate things for a bit, starting with the combo.
The Elemental Brandishes have no status on the combo and thus, the Acheron combo is objectively better than those. Bringing it back would put the two in perfect balance.
As for the charge, it's balanced with the other Brandishes pretty okay but more importantly, Divine Avenger. The two share charge times and damage values and although both are overpowered, they're balanced with each other.

While this change would leave Acheron still be overpowered, it would at least be balanced with other overpowered weapons.