iYou need maximum flexibility for your self.
Meaning you need config values for each damage dealer and damage receiver.
Two types of damage cover for most of your situations.
A flat amount (say 10) and a % amount say (20%). Obviously you need hit points for the damage.
This is for you however. Maybe you want damage overtime too.
Relaying this the players is an entirely another matter. You may not show any damage numbers at all. You may choose to show "Monster dies/in 3-5 hits from your selected creature" Or you may show only health bars and damage effects without the actual numbers.
What you want to have is good configure-ability, so when you do decide to fiddle with numbers it can be easy.
Most people hate excel but i really recommend it for configurations of the sort, when you don't have the time and patience for coding really dynamic configs.
I have a whole subset of units that have a whole lot of values to each one, I've linked all damage calculations for them to single cells. I start with a base damage number say 50 - every unit starts with it.
When you run past your logical considerations on what is how strong etc. You decide that X unit will have 5 times the base damage. Except in my case the unit has damage vs every kind of other unit. If you have to calculate these one by one each time you change them its a headache. Instead if you tied them all up, all you need to do is change 1 number and everything auto calculates.
Specifying damage vs each unit is a bit easier to manage than having one damage and assigning resistances to all other entities. You can have the machine translate that to you in terms of resistance. What do I mean? If your BASE damage is 200. And you say this unit has 150 damage vs rocks. It means the rock has 50 resistance to this unit's damage.
The more complicated damage you have the more precise planning you need to make to organize it. Obviously configuring resistances individually becomes much more convenient dependent on what you have/want/need to work with. It doesn't take rocket science. Even if you are not big at math, you just need a good plan/design. You need to understand what you want to achieve in the end, the rest is for the computer to calculate once you set the rules.
If you know your swordsman has to be 3 times stronger than your unarmed guy. and has to cost N times a much, the rest is just grinding it out into existence.
Balancing is much easier if you can translate everything to a universal value. But that takes some effort. A lot of designers just go blind in balancing stuff and fiddle with numbers manually until the game seems playable. Few have the will to go ahead and calculate everything, Its one of the reasons why many virtual economies fail. People have no idea how much comes into them and how much goes out. They think that some rudimentary concepts can do the work of exact math. But they can't.