Original D&D was skill vs armor on multiple tables – the multiple tables being to indicate that a 5th fighter had more melee skill than a 5th level wizard. Other systems use skill to hit followed by weapon vs armor to penetrate and do damage.
How could you combine this into a simple skill vs skill mechanic?
Traditional wargames have a CRT or combat results table that looks like this
The combat has a combat odds ratio calculated from the strength of the attacker and defender which selects which column to roll on and a single D6 is rolled for the result so a strength 10 attacking unit vs a strength 5 defending unit would roll on the 2:1 column.
For a game with levels the base odds ratio would be attacker level vs defender level so a 5th level attacking a 4th level would give a ratio of 5:4 and a 6th level attacking a 1st level would give a ration of 6:1.
To take class type into account the classes could be divided into three categories fighter, hybrid and magic user. The column used on the CRT could be shifted depending on the class type of the attacker and defender. Say a 3rd level is attacking a 3rd level that would produce a base ratio of 1:1. A fighter attacking a fighter, a hybrid attacking a hybrid or a magic user attacking a magic user would roll on that column. A fighter attacking a hybrid would shift the column one to the right and two columns to the right if attacking a magic user. A hybrid attacking a fighter would shift the column one to the left but one to the right if attacking a magic user.
The shifts would be:
Fighter vs Fighter – no change
Fighter vs Hybrid – one column shift to the right
Fighter vs Magic User – two column shift to the right
Hybrid vs Fighter – one colum shift to the left
Hybrid vs Hybrid – no change
Hybrid vs Magic User – one colum shift to the right
Magic User vs Fighter – two column shift to the left
Magic User vs Hybrid – one column shift to the left
Magic user vs Magic User – no shift
That could take care of class differences but still on one table.
(If the maximum column was 6:1 a level 20 Gandalf would still roll on the 4:1 table when fighting orcs up to level 3. The orcs would roll on the 1:4 table if the limit was 1:6)
(A level based game might need a CRT going up to 10:1 or even higher to compensate for this.)
The 6:1 column on the CRT might be something like:
6 – kill
5 – kill
4 – serious wound
3 – serious wound
2 – wound
1 – slight wound
The 1:6 column might be:
6 – slight wound (or even slight wound* the * signifying the ened to throw another 6 to cause the slight wound)
Armor and shields
Armor could also shift the column, heavy armor moving it two columns to the left, light by one column maybe. Shields could do the same or reduce the die roll by 1. So two heavy armored guys of equal level might be 1:1 shifted to 1:3 by their armor.
Low levels attacking an unarmored high level hero would still be 1:6 to hit.
This could either reduce effective level, shift the columns or modify the die roll. This will need experimentation to decide what best can create the Robin and Marian type effect.
Multiple low levels attacking a high level at once could shift the column by one for each extra attacker so multiple low levels at once would be dangerous to a high level unless he covers his flanks and / or uses terrain to limit the numbers who can reach him. A 6th level vs six 1st levels would attack at 6:1 and likely take out one opponent every round. Each of the attackers would roll on the 1:6 column on their own but if three could attack at once that would shift each of their attacks two to the right so they’d roll on the 1:3 instead and if all six could attack at once that would shift the column five places to the right to the 1:1 column – much more dangerous for the higher level.