If unit initiates attack, no follow-up occurs. Against foe with sword, lance, axe, bow, or dagger, if unit’s Spd - foe’s Spd ≥ 1, foe can’t counterattack.
The Sweep skill family consists of two skills: Windsweep and Watersweep (see Firesweep weapons for a separate analysis.) These skills can be difficult to understand, as they have many rules which can change due to an individual units’ stats or boon/bane. Both of these skills function the same, but work on different types of enemy units. The activation requirement is that the unit has one more speed than the foe: if not, none of the skill effects matter. Also, the skill effects only apply when the skill user initiates combat. When they do, they will not perform a follow-up attack, even if they have the required speed to do so. The trade-off for this, is that the foe cannot perform a counter-attack. This means that the skill user can effectively score free damage with the skill. They are a great counter to Quick Riposte, completely circumventing that skill’s effects by preventing counter-attacks.
Sweep skills are not suited to high-speed offensive threats who would prefer the follow-up attack, and are not suited to tanks who lack the speed to activate the skill (and would prefer to have units initiate on them.) It is best suited for units in the low 30’s for speed: not enough to lose a double attack, but enough to just edge out the opponent. Phantom Spd is a fantastic seal for use with Sweep skills, adding five speed to the unit’s speed calculations for the purposes of the calculation. Brave weapons work amazingly well with Sweep skills, allowing them to launch powerful attacks without fear of retaliation. However, the speed penalty can make it difficult to trigger the skill. It also naturally synergizes with Death Blow, both being triggered on attack. One of the best A slot choices is Fury, granting an important boost to speed.
While powerful, they have their fair share of weaknesses. A total of three requirements must be met in order to trigger the skill, meaning it can be less reliable than other skills. In the Arena, targets for the skill can change wildly between matches, due to boons, banes and merges. Skills such as Fury and Life and Death can immunize otherwise vulnerable units to its effects. Dealing with Sweep skills on the opposing team can be tricky if you rely on units with Quick Riposte to defeat enemies. It is best to take the offensive with a ranged attacker or speed debuffer to prevent activation of the skill. Lastly, giving the Phantom Spd seal to one of your own units will greatly decrease their chances of falling victim to it.