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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.

## Combos

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.

## Character types

Depending on the stats, each character is considered to be one of four different types: tank, offensive, healer or balanced.

### Tank

Tank characters have above-average HP but below-average attack and heal. Characters are considered tanks if they have HP > ATK × 3.

### Offensive

Offensive characters have above-average attack but below-average HP and heal. Characters are considered offensive if they have ATK > HP. They are the equivalent of DPS characters of other RPGs.

### Healer

Healer characters have above-average heal but below-average HP and attack. Characters are considered healers if they have Heal × 1.5 > ATK.

Healer characters shouldn't be confused with healing characters, though some companions are considered both healer and healing characters.

### Balanced

Balanced characters have average stats all round. Characters are considered balanced if they don't fit any of the other categories.

The stat comparisons are approximate guidelines, not strict rules.

## Character powers

Character powers can be used to influence combat in the player's favor. Each companion's power can only be used when it is charged up, after a certain number of turns has passed since the level began or since the character last used its power, whichever is most recent. Each of the Doctors' powers can only be used after the player has completed a certain number of combos, and has a second-tier stronger effect or a completely different effect if additional combos have been completed. Should any of the doctor's power have different effects, those doctors will be listed in both categories.

To find out how long before the power become available, holding down the charater portrait during the battle screen will display a pop up showing how long before the power become available and when the power is availalble, the character potrait will flash. When the doctor's second tier ability become avaialble, there will be a flashing outine on the doctor's potrait. Pressing the character potrait when the charater's power is availble, will show a confirm screen to use the ability.

### Bomb

Bomb characters have powers that deal a fixed amount of damage to one target enemy after a number of turns have passed since the power was used.

### Boost

Boost characters have powers that increase the damage dealt by matching a certain color of gem. When used, each gem of the appropriate color on the board at that time will be marked with a plus symbol. Any combo made that includes one or more of those gems will receive an additional 0.10 multiplier to its attack strength for each of those gems used.

### Convert

Convert characters have powers that change all gems of one color on the board into another color.

### Cure

Cure characters have powers that cure the player's team of poison.

### Damage

Damage characters have powers that deal a fixed amount of damage to either a target enemy or all enemies. This damage is modified by gem color affinities as well as any enemy-specific innate damage reduction.

### Healing

Healing characters have powers that heal the team by a fixed amount.

Healing characters shouldn't be confused with healer characters, though some companions are considered both healing and healer characters.

### Poison

Poison characters have powers that poison a target enemy for a number of turns.

### Stun

Stun characters have powers that stun an enemy for a certain number of turns, preventing the targeted enemy from doing anything for that duration.

## Enemy powers

In addition to attacking, some enemies also have powers that can be detrimental to the player. Most of those powers have a cooldown of one turn, although some enemies can take more turns.

### Bomb

Bomb enemies have powers that deal damage to the player's team after a number of turns have passed since the power was used.

### Convert

Convert enemies have powers that change all gems of one color on the board into another color.

### Healing

Healing enemies have powers that heal themselves.

### Lock

Lock enemies have powers that lock a certain number of gems on the board for a number of turns, preventing them to be moved. Note: the icon for both lock and neutralise are identical.

### Neutralize

Neutralize enemies have powers that neutralize a certain numbers of gems from the board for a number of turns, preventing them to be used in combos. Note: the icon for both lock and neutralise are identical.

### Poison

Poison enemies have powers that poison the player's team for a number of turns.

### Remove

Remove enemies have powers that remove all gems of one color from the board.

### Stun

Stun enemies have powers that stun a character on the player's team for a certain number of turns, preventing the character from doing anything for that duration.

### Summon

Summon enemies have powers that summon other enemies to the battlefield.

### Weaken

Weaken enemies have powers that weaken pink gems. When used, the pink gems on the board at that time will be marked with a minus symbol. Any combo made that includes one or more of those gems will receive an additional - 0.10 multiplier to its healing power for each of those gems used.

## Damage formula

The damage formula for a given character's attack versus a given enemy is as follows.

$\text{damage} = a * (1 + [N-1]*0.25) * ATK * \sum_{m \in M}[(1 + [g - 3]*0.25) * (1 + p*0.1)] - R$

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 N is the total number of combos made. The summation represents the sum for each combo m over all matching combos M, where g is the number of gems matched in m and p is the number of powered-up gems (from increased damage powers) used in m. Finally, R is an enemy-specific value which reduces damage dealt to that enemy by each attack by a constant amount (for many enemies, this value is 0); this reduction can't reduce the damage the attack deals to less than 1.

For example, a turn in which two blue combos are made (and no other combos), one with 4 powered-up gems and another with 3 non-powered gems, the summation would yield the result 2.75: the first combo has a multiplier of 1.75 (1.25*1.4), and the second has a multiplier of 1. A blue character would then have an attack strength of approximately 3.44 (2.75*1.25) times its ATK, and the damage dealt to the enemy would be that attack strength modified by the appropriate affinity multiplier and potentially reduced by some amount for that specific enemy.

Against single enemies, this often means maximal damage output can be 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, damage reduction is applied to attacks on a character-by-character basis; for instance, an enemy that takes 1000 less damage from attacks (R 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).

## Healing formula

The healing formula is similar to the damage formula, except it uses the team's combined Heal attribute. The formula is as follows.

$\text{health gained} = (1 + [N-1]*0.25) * Heal * \sum_{m \in M}[(1 + [g - 3]*0.25) * \max{(0, 1 - p*Q)}]$

Where, similar to the other formula, N is the total number of combos made, and the summation represents the sum for each combo m over all matched combos of pink gems M, where g is the number of gems matched in m and p is the number of powered-down gems (from enemy powers) used in m. Finally, Q represents the percentage by which the enemy's power reduces a combo's health gain for each powered-down gem used in it; this number varies among different enemies' powers.

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. That effect is significantly increased by the presence of powered-down pink gems.

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