Swiftstrike is a Keyword on Troop cards that allows the Swiftstrike troop to deal its damage first in combat.  

If the damage dealt is enough to kill the opposing troop, that troop is sent to the graveyard without dealing its own damage in return.  If both troops have Swiftstrike, they trade damage and combat is resolved as normal, but still before any other non-Swiftstrike troops in combat.

This keyword performs a similar function to the First Strike keyword in Magic: the Gathering.

Example Card: Pack Raptor

Source: Gameplay Video