When two or more characters go at it in combat, some special rules apply. The XP may invoke combat rules for other actions sequences as appropriate (such as a car chase or fleeing from the building before the bomb goes off). These are sometimes referred to as Action Sequences.
Combat works in one second Rounds. (Rounds function like turns, except we call them rounds).
Each combatant rolls at the same time, and the highest total dictates the results.
Very often combatants will do more than one thing at a time (attacking while defending is most common), and their dice must be divided accordingly.
See Task Rolls if you haven't already.
See Action Sequences if you haven't already.
A combat attack is a Contested Task against the difficulty of the attack and the target's defense. If the attack total exceeds the difficulty plus the defense, the attack is successful. On a successful attack, divide the Difference by the victim’s current base Dice + EN. The result is how much damage the character has taken – in dice. (We do not use Hit Points).
Hitting a human sized target with a physical blow, a melee weapon or even a gunshot at short range is actually easy, so the default difficulty is one dice. Hitting an actual person, however, especially one that knows he is in danger, has an inherent difficulty equal to that person's current Core Dice.
Note that the Core Dice is the Difficulty - not a defense, and no other stats or skills or the like factor into it. To use any of that, the character needs to actively defend - which supersedes the inherent difficulty.
Some attacks may be inherently harder to pull off, and may have higher inherent difficulties, but these are the exceptions ("I use my super-ninja death-strike..."). The assumption is that the attacker aims for the torso or equivalent center of mass. Specific targets (such as head shots) add to the difficulty. Such attacks are stunts (Called Shot). Long range attacks have increased difficulty (see weapons).
Size Matters
Large creatures must ignore a number of dice (the lowest presumably) equal to their size when determining Order of Action for physical actions. Conversely,small creatures may add a number of dice equal to their size for Order of Action purposes only. This rule does not apply to mental or emotional tasks.
Defense Matters
Attack totals that are not then reduced by some defensive measure are frequently devastating. This is the most common misunderstanding from new players: if you put up no defense, even the lowliest extra can clobber you. Unarmored combatants are wise to use some dice for defense (anyone can dodge - even if they have no particular skill at it).