Hmm what can be a good idea is to make every character be the same no matter the lvl and no matter the equipement.
For example: A lvl 60 sieg vs a lvl 20 sieg, they both start out with, say, 5k hp and they both do the SAME exact amount of dmg range on eachother. That way, the only difference will be the amount of skills the lvl 60 has that the 20 dosnt. That would literally be balanced. No eq mess ups, no passive skill mess ups.