Another way to finish this game quickly is by using Jill. The difference is near the end of story; 
1) When you meet Barry behind the waterfall, say "No" when he invites you to join or to wait for him 
2) Do not bother to take the MO Disk. As soon as you are in laboratory B3, just go right to the dark room (one with ceiling creature), activate the power, and then activate the elevator in the next room. Just run, do not kill the creatures if not necessary. After the elevator is activated, use it and you'll meet Wesker. Kill the Tyrant, get the lab key from Wesker's body, get into the lab B1, plug the battery, and use the flare. YOU WON'T MEET TYRANT, the heli will take you up. In consequence, you wont get the rocket launcher.

Kill Tyrant:-  How to kill Tyrant with 1 shot at the helipad (last level) run around trying to avoid the Tyrant, wait until you have 30 seconds left and always stay by the door of the helicopter someone will throw down a rocket launcher. pick it up and blow him to tiny pieces!

Weapons Select:-  NOTE: THIS CODE WILL ONLY WORK ON SOME VERSIONS OF THE GAME 

When the title screen pops up put in this code: Triangle, Square, Square, Cross, Triangle - You should hear a scream to tell you that you've done it right. 
When you start playing press Start, then Cross, Cross, Cross, Triangle then Start and you'll have all the guns including a secret Flame-thrower gun. 

Plant 42:-  I killed Plant 42 using nothing but a box of shells for the shotgun and about 30 or 40 shots from the Beretta. Here's how: enter the room, keep running, stop here and there and shoot the Plant with the shotgun. Shoot once, and run. You will run out of shells before it dies, so switch to the Beretta and keep shooting. The Plant will die once, BUT IT WILL COME BACK! Only when it comes back, it has only one or two tentacles. Walk REAL close to the Plant (there are many places where it can't reach you), aim high and never stop shooting. It will die soon. 

I found out away to kill plant 42 without using any bullets or knives. First you go to the second typewriter that has a crate, then next to the crate press X and it will ask you if you want the herbicide say YES. Then go to plant 42 and put the herbicide in the water pump by going up to the water pump press start and go to the herbicide and press X and go to use herbicide and the plant will shrivel up and die. 

Change Clothes:-  Finish the game and (Rescue the others) wait for the message "You have got the special Key". Save and start the game. Go into the room that has the big mirror in it and unlock the door at the back. On entering the closet press CROSS and change clothes.

Doom Books:-  To get the emblems from the Doom Books, examine them and turn on the side with the pages facing you. Press X and you can get the emblems. 

Rocket Launcher in Whole Game:-  Finish the game within 3 hours (don't bother to save anyone in order to conserve time). The result is well worth the effort! Only potential problem is that the rocket launcher doesn't shoot upwards or downwards. 

Secret Room:-  Collect all the mansion keys. When the keys become useless, do not discard them. When you beat the game, you will get the closet key. With the closet key you can get a new set of clothing for your character plus you can access a secret room within the mansion.

Gameshark Codes:-
Infinte time: 800C867C 0000 Any item in pos.:(Chris only) 1st pos.: 800C8784 FFxx (it's not xx) xx= 01 Knife 2nd pos.: 800C8786 FFxx 02 Normal gun 3rd: 800C8788 FFxx 03 Shot gun 4th: 800C878A FFxx 04 Colt python (red) 5th: 800C878C
FFxx 05 Colt python (green) 6th: 800C878E FFxx ( trust me those Colt's rule!) 06 Flame thower Any item in pos.:(Jill only)
0F Flame thrower ammo 1st: 800C8780 FFxx (it's not xx) 07 gernade gun (gernade) 2nd: 800C8782 FFxx 08 gernade gun
(acid) 3rd: 800C8784 FFxx 09 gernade gun (flame) 4th: 800C8786 FFxx 0A Rocket launcher 5th: 800C8788 FFxx 0B gun ammo (norm.) 6th: 800C878A FFxx 0C Shot gun ammo 7th: 800C878C FFxx 0D Colt ammo (red) 8th: 800C878E FFxx 0E
Colt ammo (green) 10 gernade gun ammo (gernade) Any item in storage box: 11 ger. gun ammo (acid) (Jill&Chris) 12 ger.
gun ammo (Flame) 1st pos.: 800C8724 FFxx (it's not xx) 13 empty bottle 2nd: 800C8726 FFxx 14 bottle of water 3rd:
800C8728 FFxx 15 umb no.2 4th: 800C872A FFxx 16 umb no.4 5th: 800C872C FFxx 17 umb no.7 6th: 800C872E FFxx 18 umb no.13 7th: 800C8730 FFxx 19 yellow-6 8th: 800C8732 FFxx 1A NP-003 9th: 800C8734 FFxx 1B V-JOLT 10th: 800C8736 FFxx 1C rusted shot gun 11th: 800C8738 FFxx 1D sqare turner

Gameshark Codes

Unlimited health:
300C51AC 004C 
Have rocket launcher at start:
D00C8784 0001
800C8784 630A 
Unlimited bullets in handgun:
800C878A 0F02 
Unlimited rockets for rocket launcher: 
800C8784 630A 
Unlimited ink ribbons:
800C8788 042F 
Colt Python:
800C8786 6404