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This is where things start to get truly nasty. Avoid this fight until you have fire weapons or items. The Maneater fight starts out with one gargoyle, which isn't so bad, until you hack away at the first one a bit and a second one shows up. The problem here is that you're fighting two monsters, both of them powerful and capable of flight, on a walkway suspended above the abyss. I had no luck using arrows on them and instead focused on separating them from each other and killing the weaker one first. Go for the tail(s) first. Severing tails removes some attacks from each Maneater's arsenal.
Summon? Yes, if you can.
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12. Armor Spider
This fight sucks. You start in a narrow mineshaft with the eponymous spider far ahead of you. It's bad enough you have to take on a giant spider but then, suddenly, it spews fire at you like it's a dragon. I probably died the most in the entire game during this fight. Eventually I got lucky, reached the spider's lair, and dove behind a wooden pillar. I spent the rest of the fight leaning out of cover and pelting that monster in the face with arrows until it died.
Cheap? Sure. Cowardly? Oh absolutely. But anything goes in Souls.
Summon? No. If anything, summoning another player will just crowd the tunnel you need to sprint down.
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13. The Vanguard
The Vanguard is a super powerful boss that you will fight if you live long enough in the tutorial. There's nothing to taking on the Vanguard except dodging when it's attacking and then going on the offensive. However for a low-level player/new player, this fight is extremely difficult. The kicker? Your reward is being killed by the Dragon God! Ain't that a kick in the head?
You also fight the Vanguard later on but by that point in the game you've probably leveled up quite a bit and the encounter is laughably easy. But man, he is a pain when you first encounter him.
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14. Old King Allant
This guy is a jerk. Not only is he super quick but one of his attacks will drain you of your soul level permanently (you don't even get it back after you win). There's a super cheap way to beat him involving using the Thief's Ring and just arrowing him to death from the fog door but if you want to do this fair, a shield and something pokey is probably the best way to go. Lock on, circle him, wait until he attacks and then, after he misses, stab him and bounce away. The key here is not to let him ever get too close to you. His attacks don't hurt too much but that soul level drain can put you in a nasty bind that affects you for the rest of the game.
Summon: Yes, if you can.
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There's a lot of ways to fight this baddie, which contributes to why this is such a difficult fight. There are three floors here and there's a different strategy for each one, so it can be a bit overwhelming initially. The easiest strategy for me was to rush down to the ground floor and attack his bloody cleaver. After he takes enough damage, like Tower Knight, he kneels forward and allows himself to be attacked for extreme damage. Take along a lot of healing items if you use this plan.
Summon? Yes. Having a buddy for this fight makes it so much easier.
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16. The Penetrator
The Penetrator is a tall knight wielding a ludicrously large sword. If you're playing a melee class, this is probably going to be one of the hardest fights in the game. Learn his patterns. You will probably die a few times doing so but, really, it's worthwhile to not waste the supplies while you're learning when to dodge his attacks. When you've got those memorizations down pat, take him on—preferably with a shield and spear—and be sure to use arrows as well. Magic users shouldn't have that much difficulty with this since Penetrator is weak to fire. Just keep your distance and flame on, ya pyros.
Summon? Depends. Having someone else along will make this fight less of a pain but one of the most satisfying things to do in the game is beat Penetrator on your own.
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17. The Old Monk
The Old Monk fight is really cool. And also awful. Essentially this battle has you taking on a player that's been summoned to your world. If this is your first time playing Demon's Souls, that is going to suck because players who have played the game for years, and enjoy being predators, will hover around this area on purpose just for the sake of giving new folks a hard time. The best thing to do here if you're having trouble is log off PSN and then fight the AI Monk, who's actually pretty easy to kill. Roll around a lot and stab him. Be sure not to stay away from him too long since he can back away and heal himself.
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From hell's heart, Flamelurker, you asshole. This fight is easily the most difficult and frustrating encounter in all of Demon's Souls. Flamelurker is both fast and powerful and he will rip you a new one if you screw up just once. If you're playing melee, a good deal of preparation goes a long way in this fight. Grab some flame resistant armor, a strong weapon, and always try to stay in front of Flamelurker. Think of it as a boxing match and stick to the center of the room. If you let him back you into a corner, the fight is over. Accept that you are probably going to die until you get lucky enough to have a fight where everything goes your way and you land the killing blow. Try not to get disheartened by this one. If you can beat Flamelurker—and you can—you're capable of beating every other boss in the game.
Summon: Yes, for the love of god, summon if you can.