How to Destroy A Submarine
This is your first boss. Collect the weapons and proceed to destroy the guns on the submarine. In total there are three guns – as soon as they are destroyed, the submarine will go to the bottom.
The left lower gun shoots heavy shells flying along the arc (hide from them under the wooden bridge), the upper weapon is the machine gun, the lower right weapon is the flamethrower (also use the bridge when the flamethrower shoots from the top down (and the submarine moves from side to side) or jump to the upper and lower levels of the bridge, if the flamethrower is conducting horizontal fire.
Focus on the lower left weapon. Destroy it as soon as possible, because the longer the fight is delayed, the higher the probability that you will be exposed to fire from all the guns. Further destroy the flamethrower, then deal with the machine gun, the shells of which are not so difficult to shy away.