I would recommend you watch the online tutorial videos they cover this. Basically... Attach a Transform Logic piece to each of your enemy objects, set the Linear Velocity to -1, Affected Assets to Enemy and lastly Affect Operation to Multiply. Now when your enemy is traveling at a linear velocity of e.g 10 in the x direction comes in contact with another enemy it will collide with the Transform piece, its linear velocity is multiplied by -1 which makes it -10 in the x direction i.e opposite direction.
