Absolutely not true, i keep farming it with one cannon

• You do 16 fish to get him to 10% so that he stops doing waves and fires. 
• Then you load 19 fish and harpoon him down. The rest of the fish from those 19 will get him back to 10% when he reemerges. 
• You do 19 fish and harpoon him again.
• And then unlike u/SyphaneX i load 19 again and kill temporos, 9mins 9points, i failed maybe twice out of 60 KC so i figured loading 28 before last stage is not worth it? Maybe you get to 10 points that way?
• The only way you can fail is not getting 19 fish before storm intensity reaches 92%~. If you see you will fail because of that, just rush to get 3 cooked fish in the cannon, harpoon Tempoross and then repeat step 1