The combat mechanics of Doctor Who: Legacy involve matching colored gems in combination with the abilities of a team of characters. Combat is turn-based, allowing the player to take his or her time making a move. To make a move, the player selects one gem on the board and drags it around the board for a short period of time, swapping it with other gems as it moves.
- Main article: Character types
Depending on the characters' base attributes, DWLegacy Wiki classifies each ally character as one of four different types: tank, offensive, healer or balanced.
- Main article: Character powers
Character powers can be used to influence combat outside of attacking, such as by changing the colors of gems on the board or by inflicting the poison condition. Both ally characters and enemy characters can use powers.
- Main article: Gem colors
The damage dealt by an ally character's attack is affected by gem color affinities. An ally character deals twice as much damage when its color is strong against the enemy, while it deals half as much damage when its color is weak against the enemy. When targeting an enemy, arrows will indicate for any allies whether their attacks would be strong or weak against that enemy.
- Main article: Perks
Perks are passive benefits conferred to the player's team during the game, such as bonus damage or damage resistance.
A combo is a group of gems composed of horizontal and vertical lines of three or more gems of the same color in a row. After the player makes a move, all combos on the board will be cleared away; any empty spaces on the board are replaced by gems from above (new gems will also fall from above), then any new combos on the board are cleared, and this process is repeated until no new combos form. Any characters on the player's team that match the color of the combos will get an attack for the turn; a combo of pink gems causes the player's team to gain health based on its combined Heal attribute. If five or more gems are cleared in a single combo that triggers an attack, attacking characters of that color will attack all enemies.
The strength of an attack for a given character is based on its ATK attribute. A group of three gems matching a character's color results in that character getting an attack strength equal to the character's ATK. Each additional gem cleared in a single group of gems adds 0.25 of the character's ATK to the attack strength. The same is true for healing, except it uses the team's combined Heal attribute instead of a character's ATK.
If multiple combos are made in one turn, each additional combo beyond the first made in that turn will add an 0.25 combo multiplier to the strength of each character's attack, as well as to the amount of health gained by matching pink gems.
Combo strength formula
Combo strength represents how much an individual combo contributes to the strength of an attack or heal (whichever is appropriate). The formula for combo strength is as follows.
Where stat is the appropriate stat used for the combo (the team's total Heal attribute for pink, the attacking character's ATK otherwise) and g is the number of gems in the combo. Boosts are the total of the boosts that have been applied to each individual gem in the combo; for example, if each gem in a three-gem combo has been boosted by 10%, the sum of the boosts is 30% (that is, 0.3). Similarly, Weakens are the sum of the amounts by which each individual gem in the combo has been weakened. It is possible for an individual gem to be both boosted and weakened.
The damage formula for a given character's attack versus a given enemy is as follows.
Where a is the affinity multiplier (2 if the gem color of the attack is strong against the target, 0.5 if it's weak and 1 for neutral affinity; if the weakness enhance perk is applicable, these become 2.5, 0.25 and 0.9 respectively, though the last only applies to black and yellow attacks) and N is the total number of combos made that turn. In this context, perk boosts refer to those boosts granted by perks that increase attack strength (specifically: color enhance, technical enhance, number enhance, increase damage and combo master). The summation represents the sum of the combo strengths of each combo m of the appropriate color. Finally, D is the enemy's defence, which reduces damage dealt to that enemy by each attack by a constant amount (for some enemies, this value is 0). The damage dealt by each attack is always at least 1 (even if the attack strength is 0 or the enemy's defence is higher than the damage that would otherwise be dealt).
Against single enemies, maximal damage output is often achieved by making many small combos of three gems rather than fewer large combos. For instance, two combos of three gems in the same color yield a final multiplier of 2.5 (1.25*2), whereas a single combo of seven gems yields a multiplier of only 2 (1+0.25*4). Intuitively, this is explained by the fact that each gem in a combo of three gems contributes one-third of the character's ATK, whereas each additional gem only contributes one-fourth, in addition to the higher combo multiplier obtained by matching more combos.
Notably, defence is applied to attacks on a character-by-character basis; for instance, an enemy with a defence of 1000 (D value of 1000) will take 2 damage (1+1) from two characters with neutral affinities who attack for 800 each, rather than 600 (had the reduction been applied to the total damage).
The healing formula is similar to the damage formula, except it uses the team's combined Heal attribute. The formula is as follows.
Where, similar to the other formula, N is the total number of combos made that turn and the summation represents the sum of the combo strengths of each combo m of pink gems.
Similar to attacks against a single enemy, maximal healing is achieved by matching many small combos of pink gems rather than a large combo of many pink gems.