Dungeons of Sunnydale has a unique (but probably subconsiously based on other, similar combat systems) combat system made up by the author in about ten minutes. It consists of various dice rolls and skill checks.
When a combat begins, the two combatants' agility stats are compared. The combatant with the higher agility stat can act first.
The battle menu consists of a variety of options, as well as the Player Character's most important Primary Statistics: HP and MP. The options are as follows:
Attack: This option is the most basic battle maneuver that can be executed during combat. If this option is selected, the Player Character will attempt to strike their opponent with whatever weapon they're holding. Whether this strikes or not, and how much damage it does, is determined by an Attack Roll.
Special: This option allows you to use any ability which you have learned; for instance, during 0001's partnership with the Shopkeeper, he learned the Special Ability Double Attack, which allowed him and the Shopkeeper to attack at the same time. This menu also allows you to use Color Synergy in battle, although it is unknown what the effect of this is.
Flee: This option also hasn't been seen, but it can be assumed that it performs some sort of skill check against the opponent and then ends the battle.
Items: This option presumably opens the Player Character's inventory for use within combat.
An attack roll starts with a base roll of a die. The number of sides of the die varies from creature to creature, but for the Player Character a single eight-sided die is used. This roll, known as the base roll, is then put aside for future use.
The next step in the attack roll is to add in any modifiers. to the roll. These can have several different effects, but the most common one is a bonus. Every character with a Strength stat above or equal to their current level gets a Strength bonus to their attack. This bonus is calculated by dividing the character's Strength stat by their level and rounding up. This number is then added to the base roll.
One modifier unique to Player Characters is the Color Synergy bonus. A Player character gets a +1 bonus to their roll if their equipped weapon, or the opponent they are facing, or both, has the Player Character's exact Associated Color on its sprite.