Silent Hill

It is quite funny to kill off this boss.
Shoot him with the shotgun and he will dive down. He will stay down for a while and sometimes bump into you when swimming past you.
As soon as he reappears shoot him again with all the shotgunpower.
Watch out, so that you are not too close to him when he rises up again, as he ponches around him when rising.
But beeing too far away is also bad as your shots don't hit him hard enough then.
So let him rise and then get to it and shoot!

After about 11 shots from the shotgun Leonard is down.

When you use the pistol it takes longer, much longer.
You can also use the submachine gun, but beware, the ammunition is rare and gone really fast.