Deadly Strike and Deadly Shot are actually the same power. The original name of the power, given to the Omnicron Snipers (who use handguns), was Deadly Shot. When the power was later reused with the Minions of Utgar, who use battle axes, the name no longer made sense, so for them the power was renamed strike instead of shot. This power would later go on to used by other figures, under both names.
When an attacking figure with the above abilities rolls skulls, the number of skulls rolled is immediately doubled. For example, 1 skull rolled counts as 2, 2 counts as 4, 3 counts as 6, and so on. The amount is doubled BEFORE defense is rolled, so if 2 skulls are rolled for attack, then a defense roll of 4 shields will block the attack, while a roll of 2 shields will result in 2 wounds inflicted on the defending figure.