It really depends... Laser is not a 1 shot deal. Just like Electrolysis isn't a 1 shot deal. They both work under the exact same principles, causing damage to the folicle so that the hair is unable to re-grow. And as bklyn mentioned above, for both laser and electro, the hair folicle has to be in the same state of growth, in order for it to be effective. And depending on your type of hair/skin, laser may or may not be effective.
Laser works by heating up the hair folicle to a high enough temperature that it damages the cells that grow the hair. So the darker your hair, and the lighter your skin, the more heat will be absorbed by the hair folicle, thus giving you better results. But you'll prolly have to get several treatments in order to get the majority of the hairs. And if it's an area that has blonde hair mixed in with dark, you'll prolly still have to get electro to finish up the lighter hairs.
There are two types of electro, well 3 really. Thermo, Galvanic, and Blend. Thermo basically just gets really hot, and uses heat to damage the cells. Galvanic uses the process of electrolysis to create acid (lye) out of the salt/water (sweat) in your pores. And blend basically combines the two.
Personally... if you have dark hair and pale skin, I would do a full cycle of laser first (thats like 4-6 treatments, not just 1 and your done), and then clean up the blonde hair remaining with electrolysis. Assuming you want a completely smooth area with no hair growing at all.