The Del Lago is a very fast and sneaky boss. The main problem is the terrain. You are stuck inside a boat, in the middle of a lake, with nothing but harpoons.
How do you overcome this obstacle?
11 Answer
Well, no magic involved here. Try to avoid the obstacles when he pulls you, try to hit him with the harpoons at the same time, by aiming a little higher. When he seems to have disappeared and you auto switch to aiming mode, wait for the signal about his location and shoot as much as you can. Aim a little lower to be sure to hit him.
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