Huge Plant

The main problem with this gigantic plant (or whatever that is) is that
most people think that all that matters is hitting it with their cannon,
without even noticing that the not-so-fearsome beast actually has an
hidden eye, which is shown at the beginning of the battle. Obviously, you
should be hitting that part of the body, instead of focusing your attacks
in random points.
Now, the question is: how can I make the eye appear? It's simple, that
monster only shows the eye when it doesn't feel threatened, I mean, when
you're so far that you can't hit it. So, simply move your ship away for a
bit, until you can see the eye. Then, quickly set a new route in the
direction of the beast and hit the eye ONCE (you can't even do it more
than once, in case you're wondering).
Repeat until you defeat him, which doesn't take much time and works even
better if you trace the smallest possible route.


Flying Fish

As you can easily notice, this huge creature has some odd eyes in both
sides of its body. Hit them and he will be down in no time. Sorry, but
there's not much else I can tell you about this enemy, and if you can't
even follow these instructions, hitting the balls for extra health as I
previously told you, maybe you shouldn't be playing this game.


Ghost Ship

Unlike people may think, this battle is easier than it seems, if you know
what you're doing.
First, instead of looking at the console's upper screen, which simply
shows the location of the eyes stuck on the ship, focus on hitting the
green balls that are thrown at you, which are the only real threat here
and may range from one up to four at each time.
Then, when there are no balls coming in your direction, use your cannon to
hit the eye which is closer to you, no matter which one it is. If one goes
down, start hitting the next one. Eventually those eyes will be back, but
you shouldn't care about it. Instead, continue following this strategy,
first hitting the balls and then focusing on the eyes, and you'll later
notice that the ship will appear to be going down, literally.
When that happens, and you can't seem to find any other eye, start looking
at the upper screen, as it displays the position of the eyes. The most
difficult to hit are placed in the upper part of the ship, and only tend
to appear for a limited time. So, you should be more careful about those,
taking care of all others before focusing on the stated two, which may be
harder to hit.
When all the eyes are down (which will be a short and easy battle, if you
follow my instructions), this enemy will die and you'll proceed in the
game.