he shot his plasma at the hulk's forehead, thinking to himself that he would attack hulk's "mind," and the fact that he was deriving his power from the ambient gamma radiation within the hulk as well as focusing the beam tightly on his forehead gave the hulk pause.
havok's powers likely aren't strong enough, fully charged or otherwise, to stop doomsday. his power accumulates over time, not from some mental strain he puts on himself to charge up, so a full charge is a full charge whether doomsday's half a mile away or right in his face. if he can concentrate it into one big outburst, it'd likely be the first time he ever did it on purpose. and still not enough.