Location: Midwich School Boiler Room
Tactics: This can be an easy fight or it can be the most difficult in the game depending on what gameplay level you are playing at. Easy mode can be done in one round of waiting for the Lizard to open it's mouth and fire off a shotgun blast into its mouth. Normal I believe takes 2 or 3 shotgun blasts. Hard mode is ridiculous! It took me so many tries to get this bastard down it's not even funny. The worst thing about this is that if the Lizard swallows you it's instant game over. But getting close enough to shoot him in the mouth when he opens it in such a cramped room can be very difficult. Take your time and shoot it as much as possible with your handgun. You'll see little bits of blood come off, I'm not absolutely sure if this damages it, but it seems to induce it to open it's mouth. Just take your time, and run around that bonfire as much as you need to to be safe!
Larva
Location: In the Mall (Shopping Plaza)
Tactics: This fight is very easy, just shoot it with your shotgun or newly acquired rifle.
The Fight
The Moth
Location: Outside of Alchemilla Hospital
Tactics: Fairly straight forward - shoot it. People have mentioned hiding behind a water tank, but it's such a cramped space that if you move much you lose track of where the moth is. I usually just brave it and shoot it down and use a health drink or two afterwards.
The Fight
Cybil
Location: Merry-Go-Round - Lakeside Amusement Center
Tactics: A difficult fight either way but choose a desired outcome A) Save Cybil or B) Kill Cybil. This is one of the major factors that determines what ending you get so decide carefully.
A) To Save Cybil you must have gotten the red liquid (aglophotis) from Kaughman's office in the hospital. You have to avoid her steady shots while also managing to get a couple hits in on her too. At a certain point during the encounter she will run out of ammo and toss her gun. You'll notice that she will kind of walk away from you and then back. You can see a big sac on her back (like the ones on the nurses) and you guessed it, you must then use the aglophotis on her. This will "save" her and you can watch the yucky squirmy thing come out of her! Ew!
B) To Kill Cybil - well -pretty self-explanatory. Just kill her.
Alessa/Cheryl
Location: Nowhere
Tactics: While this might not be an actual tactic - I've found myself using it quite a lot in the past because this is a pain in the butt fight. If you unload ALL of your ammo before heading down the stairs to the final fight - you can automatically win. As long as you have no ammo and only melee weapons remaining - you will have no way of being able to kill the boss - giving you an instant win! I have no idea why Konami did this, but it's a nice loophole if you don't feel like putzing around with these 2 bosses for the millionth time.
To seriously kill her - basically stay away from her lightning bolts and stay away from her forcefield. Just keep shooting and running, shooting and running!
Samael
Location: Nowhere
Tactics: The same tactic that I mentioned for beating Alessa, also works for Samael. So use it if you feel so inclined. For a serious fight - avoid his lightning and make sure to take advantage of the time you get while he's charging attacks to get some good shots in. Use your most powerful weapons, if possible, and he should be down fairly quickly.