Faustus and Marcus have already been mentioned, so I will not bother with them.
Instead, I shall inform the uninformed of the methods of defeating Sebastian, Magnus, and the Sarafan Lord.
Sebastian; (Without the Soulreaver cheat) -
You must simply lure him into the mist that appears around the room and block his attacks/back away from him whilst he is consumed by it.
After a degree of his health has been eradicated, he will jump to the side of the room out of your reach and will run around it with cries of "Getting Dizzy, Kain!?!" As he does this, a large pillar of energy appears in the centre of the room. The idea is to make sure that this is between you and Sebastian when he stops and shouts "Here I Come, Kain!!!" If performed correctly, he will jump into the energy cylinder in his attempts to reach you.
Repeat this until a cutscene takes place, in which Sebastian bellows "John, Raise the Dias!" A man will then operate a lever and Sebastian will leap onto the dias with the threat of "I will destroy this precious Jewel!" Leap up onto it with him. Sometimes Sebastian remains on it, others he jumps off. If he stays on, block his attacks, pick him up, then throw him off, if not then simply possess "John" and operate the lever on the right. This should cause mist to appear around the Dias and Sebastian will say "No one beats me with my old trick!" (or something along those lines). Sebastian will then Join you on the dias and attack you, the idea is to block his attacks, wait for an opening, then pick him up and throw him into the mist (If you're really sadistic you may hit him up to two times before throwing him off as with any other civilian, however, three is too many and Kain will merely drop him to the floor). Repreat this until he has no health left and that should be it.
(With the Soulreaver cheat) -
Simply bosh him until he jumps up to the edge and runs around the edges. Then do as before until he jumps onto the dias, you can once again just bosh him and it will hurt him, however, if yopu pick him up you can hit him with the hilt of the soulreaver twice and then cast him into the mist, dealing more damage. This is, I think, the more fun of the two ways of killing him.
Magnus; (With or without the soulreaver cheat) -
You must run directly behind the statues and hide in close to them. Magnus will then incinerate the statue in front of you instead of you.
Then you must get to a point where there is no statue in front of you (I find it is helpful to hold R1 whilst facing him behind the statue first and then to dodge out from behind it quickly) and you have to knock him off his platform with the telekinesis power which you earlier acquired. He will fall into the water, and you have to hide behind another statue since he will then jump out and attempt to incinerate you again. Repeat this three or four times until you get a cutscene in which he says "Water is not my friend!" and he runs out through some chained up doors which he breaks open with the greatest ease. Follow him and jump down onto the platform at the centre. The idea here is to get him to charge at you whilst you are standing in front of one of the statues and get him to run into their weapons, knocking them off and revealing a telekinesis button behind them. You can either do this to all of them and then activate the buttons or activate them as you remove the weapons. Once you do this, there should be a cutscene in which the wreckage around the area should repair itself revealing it to be a statue of Moebius, the git.
Make magnus charge into his staff, and knock the bottom of it away and then get him to charge into the infinity symbol beneath the statue. (for those who don't know, a sideways 8 is what infinity is, but I'm sure you all do know.) That is that, this boss is defeated!
Sarafan Lord; (with or without the Soulreaver Cheat)
He will fire three shots at you. Jump the first, stay where you are for the second and jump the third. Then he will raise the soulreaver into the air and charge it for another three blasts (if he is being a git and is on the platform two away from you, then do not worry, he will simply shoot at you again.
You should shoot him with the telekinesis, then jump attack him. This will knock him off the edge.
After a brief cutscene in which Kain shows that he is clearly a madman, the Sarafan Lord will charge at you. You must block his attacks and dodge his nasty red attack. Block until your rage bar is filled and do the right thing and burn him with immolate! Repeat this until you get another cutscene throughout which we all wonder as to why Janos neglects to use his wings when he should, and we watch Kain gets what he wants.
Now you will be in the centre of the area. Block the Sarafan lord and dodge his nasty red attacks until either your Fury attack or, preferably, your Berserk attack is ready to be unleashed. Use whichever one you want and use it again if you have to, but you should defeat the Sarafan lord in two or three berserks. Then watch and be amazed as we stare at the sexiest final cutscee you've ever seen and which sends a shiver down your spine as our old friend Kain finally gets what he deserves. (watch it to see what I mean).
At first, you are in a small circle room with two doors. All you have to do is face the Boss and hold R1 + L1 (Attack and Block). Wait for him to attack.
He has four different types. In the first, he does a five or six hit combo. In the second, he can punch, then kick. In the third, he will just kick. In the fourth, he jumps back with a red glow and lunges forward.
When he uses his first type of attack, wait until he punches a third time, then release your block and attack. Repeat this about five times to continue.
In the next room you must use the mist form to sneak up to the furnace and burn him. To do this, you must first find the two furnaces with fog in front of them.
They are on opposing sides. When you first enter the room, one is to the left and the other is to the right.
Run around until he jumps to one of the two, then sneak to the corner and turn mist on. Slowly walk to the switch and fry him. Finally, he will jump around and lunge at you. Just run around until he lunges.
This will cause him to eventually fall. Run up to him and use a combo. This must be done about three or four times in order to defeat him.
Keep blocking and sidestepping his attacks while finding open spots for attack until you get about one quarter of his life down. Then, he will run off and enter a room will large heaters and mist around them.
When Faustus jumps on to a heater, proceed towards the beginning of the mist trail that leads to the heater and pull the switch to hurt him.
Keep repeating this procedure until he jumps up on to a chain, then run to a corner of the room and stay there until Fautus jumps down.
He will now try to jump and kick you from far away. If you sidestep his attack, he will hit the wall and will be vulnerable for a few seconds. Keep repeating this until he is dead.
Hint: Defeating Marcus:
You will fight Marcus in a cathedral, on the bottom floor first. On the left and right sides of the room are switches, one on each side. These switches activate bells which disrupt Marcus' concentration and leave him vulnerable for a short time. Hit one switch, then run to the other switch, hitting Marcus on the way.
Continue this until his "puppets" disable the bells. Go up to the second floor. Marcus will just run around until you kill him.
Use the jump ability to alternate between sides, hitting him on the way. Once he dies, you obtain the Charm ability.