This mission is fairly straight forward. You will have two minutes to light 25 torches with your swamp skiff's flamethrower. In order to get fuel to arm the flamethrower, you have to run over piranhas. Each piranha gives you one shot, and you can only hold five.
Don't go after individual fish, instead, try to run over multiple fish at a time. Press or to fire your flamethrower when aiming at a tiki-torch. When all 25 torches are lit, the gate will open, and you can drive through it and collect the treasure key.