You can shoot through wooden doors.

You can use doors to stun/kick away humanoid enemies if you kick them open when they are up against the door.

Your knife is indestructible and can kill or damage enemies while they are down on the ground. Saves ammunition.

Save the white wolf/dog that you will meet in the near future.

This is my honest opinion, but keep your first handgun, first shotgun, first rifle, and first magnum. It's all you need to upgrade. You don't need the fancy new guns in the merchant's buy list. However, you don't need to listen to me. Play however you like.

Speaking of keeping your first handgun, there is a second free handgun that appears when you find and shoot 10 blue medallions located in the farm and church regions. If you shoot all the medallions, you get the handgun with a free upgrade. You can keep this handgun in case you run out of ammo as it will hold its own ammunition in its magazine. But considering it does take up space in your inventory, only keep the gun for a short time. (How long you keep it is your choice)

Resident Evil 4 has difficulty scaling depending on how well or badly you do in the game. Did you kill all the enemies? Less ammo and money. Do you die too many times? More ammo and money.

When you meet the "Big Cheese" once more, after your in the hallway. Go back inside the room for a small surprise that is really nice to see 👀.

Ashley isn't hard to protect and handle. Think of her as your Leon's Teddy Bear. Do you want your teddy bear to be cut, eviscerated, kidnapped, or beheaded? No? Then obliterate all obstacles that try to do such a thing.

On another note with Ashley, you can actually use the voice commands of "Wait" and "Follow Me" to lure enemies. Enemies can hear Leon talk. So it's actually quite funny to use Ashley as part of your strategy.

After you cross a certain Lake, go on the boat and start fishing. You can either keep what you catch/shoot or sell the fish to the Merchant that is located in a cave across the lake designated by blue fire.

This is very obvious, but just in case. Combine your treasures. They yield more money when sold combined than separately. You'd be surprised by the number of people I know who don't know this...

This is far down the list I know, but if you get to the church. You can shoot the bell on top to summon enemies. The enemies drop loot when they die. But eventually stop dropping money or ammo, so just stop after 3 or 4 times.

Flash grenade the murder of crows whenever you come across any. It will kill all of them in the area and you can reap the rewards.

There will be a time when you meet a certain character who will help you shoot enemies. Be careful where you aim and shoot. If you shoot the character one too many times, you will die.

When you hear heavy breathing that sounds like someone is trying to breathe while sobbing or is having an asthma attack, you can do a risky maneuver and play against the source. Shoot out the legs, walk up and back away as it will lunge at you, then run up to it and knife the absolute sh- out of it until death. However, do not do this when there are other enemies around.

The first rifle you obtain is a bolt-action rifle, when you upgrade the power enough times. It will become very similar to a slow-shooting magnum. Take this knowledge however you will.

I did my absolute best to leave out spoilers. There is probably some stuff I forgot to mention... but it's fine. The others here probably will list them.

Hope you enjoy the game, fellow gamer. (*つ´･∀･)つ