If you do not yet have the Tek Skiff, use the Tek Mek cannon and a blood stalker to knock it down. Depending upon your max level, the mek may not take it down in one shot. My wild max is 300 so I'll use a rocket at first to deal the difference in torpor and finish it with the mek cannon. Keep in mind that rockets, cannon, and Tek Mek deal splash damage so it will not come out to a perfect tame. Use a long neck to shoot and lure the whale. Once it's facing you from a reasonable distance, shoot the rockets to deal the difference in torpor and the mek can finish it. The base mek cannon deals 25k torpor. If you have a blue print, it will deal more in one hit. I personally recommend the mek over the skiff as it's less likely to run away with mek vs the cannon and cannon ball method.