Double Strike: An isolated, static ability played after the attack end POP that means, "You may choose a unit that this unit can attack. This unit attacks that unit. Play only if this unit just attacked and only if it tapped for that attack." The unit must tap for the first attack, not the second. If it untaps, it may double strike again. Double Strike doesn't allow you to attack your own units.
Focus
Focus X: A cumulative ability triggered by the initial dice roll that means, "When you roll at least one natural five with this unit's attack dice before re-roll, this unit gets +X power for its next attack." Only the first natural "5" counts. A five rolled after re-roll doesn't matter. Focus applies only to dice rolled for an attack, not to dice rolls for abilities like Retaliate. Accuracy can't increase the die roll to a natural "5."