I don't think Bishop can absorb plasma. Just as he is vulnerable to bullets bishop is vulnerable to fire. Plasma is a solid that he can't absorb.
That would mean he would have to use his energy powers both defensively and offensively to have any effect on Havok.
Also, Havok has the upper hand of being able to devastate the surroundings in a hurry. Look at the pic i attached. Or see what he does in the books.
Bishop would really need to wield that power like an expert to win.
One drop of Havok's plasma would disintegrate whatever it touches.