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In true multiplayer tradition, basically everything moderately good counts, and Dark Souls is no exception. You'd be hard pressed to find a situation where the opposing player doesn'tcall you out on using these.
Homing Crystal Soulmass, a spell that surrounds your character with orbs that attack anything that gets too close to you. The breaking comes in that the Soulmasses are incredibly powerful if your magic stat is high enough and all five orbs hit. Only one sorcery is more powerful and it only has 4 castings as opposed to Homing Crystal Soulmass's 10. With an Intelligence stat of 50 and the most powerful catalyst in your possession, the spell can wipe the floor with every end-game boss in just a few castings. It and its downgraded version are also the only offensive sorceries that can be accurate without targeting, helping immensely with invading players who wear the Ring of Fog, although a patch reduced the range at which the Soulmasses trigger against such an opponent.
Lightning Weapons do considerable damage of a type that most armor provides little protection against and require no stat investment past whatever is needed to wield the weapon. These characteristics make them the preferred weapon type for low-level invaders, as well as players who prefer investing heavily on other stats.
Wrath Of The Gods, a shockwave miracle that has next to no start-up, does huge damage and is nearly guaranteed to stunlock opponents who stack poise (players using light armor don't suffer nearly as bad because they get blasted off their feet with invincible recovery frames on the first shockwave).
High Poise armor can be this against players using lighter weapons such as daggers.
On the other end of the spectrum, using the Wolf Ring and high poise armor often becomes a survival tactic in multiplayer due to the sheer amount of weapons that can stunlock opponents at 52 poise and below in a single hit (greatswords and even basic hammers come to mind...). The Claymore in particular is a heavy offender, since it blends high damage with the right stats, is lighter than most weapons of its category, and swings fairly fast. Unless you know when and how to get out of a stunlock, you're going to have a hard time fighting an opponent who knows just how good those weapons are.
Tranquil Walk Of Peace, a miracle slowing enemy movement speed to a crawl. Was pretty much universally hated before its nerf — now it's lamented how useless its reduced range makes it. The Stone Greatsword produces the same effect, and it's made even worse since it's a weapon with fairly fast swings and high stunlocking potential.
All of the Black Knight weapons have a ridiculously broken aspect in one way or another, with the common one being additional damage against demons (this turns nearly every Lost Izalith enemy into cannon fodder, in addition to several early bosses who become mooks later on) and excellent strength scaling. To wit:
The Sword; obtainable early on, high damage for its weight and requirements, and, like other weapons of its category, swiftly kills players that underestimate its power and are not used to breaking out of a stunlock.
The Greatsword; also obtainable early on, immense raw power, and some of the most intimidating movesets which incorporate launching opponents in the air. Don't let its slow swing speed fool you: the moment you get hit by it, expect your health to drop really fast.
The Greataxe; while it's obtained far later than its brethren, it's extremely powerful, and it swings much faster than other axes, allowing for quick combos that stun enemies before they can break out of it. If that wasn't enough, it swings in wide arcs, effectively dead angling unsuspecting players (see below for an explanation on dead-angling). A patch significantly nerfed its actual damage output, and its swings are fairly telegraphed. However, it's still one of the more dangerous, easy-to-use weapons out there, if just for how versatile the moveset is.
The Halberd; undisputably the most broken of the four, it combines early accessibility with incredibly high damage output (far beyond the alleged maximum of 500~), fast attacks if they connect, deceptively long reach as with other pole weapons, and ludicrous damage modifiers upon clashing with an attacking opponent at the same time.
The Zaphander/Chaoshander Build. Just enough Strength to use the Zweihander sword two-handed (16), upgrade it to +5 Lightning (for the aforementioned armor piercing) or +5 Chaos (for higher attack ratings), combine with Havel's Armour (highest poise) or Giant Armor (highest physical defense and second-highest poise) and a Grass Crest Shield on your back for stamina regen. You suddenly have a Mighty Glacier that can keep swinging forever, with armor that negates the disadvantages of the sword's slow swing speed. Mix in a couple of rings, you've got a Lightning Bruiser. This can be achieved before level 30. The player can then put the next 50-100 levels purely into Endurance, to boost stamina and equip load, Vitality to boost health; and a dash of Attunement to use Great Combustion. Again, pretty much universally hated for being generic, bland, stupid-easy to play and frustratingly popular. A number of its components were nerfed in v1.06, but it can still be very annoying.
The Giantdad, a popularized version of the Lightning Bruiser build above, and by far one of the most used and hated variants: it combines the bare minimum of stats to use Giant's Armor (the highest physical defence in the game), the aforementioned Chaoshander to deal very high damage, the Grass Crest Shield for a huge increase to stamina regeneration, and most importantly, the Father's Mask and rings to make the build light enough to give the player full mobility. It all comes together to create a Lightning Bruiser that's very hard to hit, can tank a surprisingly high amount of damage, stunlocks you easily, and dishes out the pain. Every. Single. Swing. note Just don't tell anyone you raised that dexterity stat.
Abyss spells, specifically Dark Bead (a fast-casting, multi-projectile spell that easily lands multiple hits up close and spreads over akin to a shotgun), Black Flame (Great Combustion dealing even greater stamina drain), and Pursuers (a Homing Soulmass variant that's significantly more persistent, and isn't stopped by the Silver Pendant, which is made to counter Abyss spells). Since you can only obtain them through Artorias of the Abyss, they can also be considered as Bribing Your Way to Victory.
The Greatshield of Artorias for defense. Even with its average elemental defense, this shield boasts the highest stability in the game, blocks nearly every status effects (save for Curse) and doesn't require as much strength to use as Havel's Greatshield. When equipped, a mook's attack can barely chip one-tenth of the player's stamina bar and most boss-level physical attacks (minus elemental attacks and some very few particular special moves)- Artorias' triple backflips, Smough's hammering, Ornstein's spearing, you name it - which can break defense in two hits, become scratches on its surface without costing much endurance. When equipped with Havel's Ring for equipment load increase and Cloranthy Ring for stamina regeneration, the player can take on literally every hit in a boss fight while slowly chipping off the boss' health bar, making the fight a battle of attrition needing little efforts and skills to win. The fandom has a good reason to believe that had Artorias been with his greatshield in his encounter with the player, we would have been toasted.
While its stability is not as high as the Greatshield of Artorias, the Silver Knight Shield comes very close. In exchange, however, it is only a medium shield, thus it has lower weight and is still able to parry. It also has a better lightning damage reduction than the Greatshield of Artorias, which will mitigate other pesky gamebreakers.
In terms of damage per second, there is nothing quite as overkill as using fast-hitting weapons like the Falchion, which has some of the fastest swings in the entire game, buffing it with a powerful miracle like Sunlight Blade, then applying Power Within on yourself. The result is a frighteningly powerful Glass Cannon that can keep spamming the attack button and even decimate bosses within the buffs' durations, if not seconds, and will usually down a full-health player in no more than four unblocked swings.
Some of the more aggravating PVP techniques are also laughably easy to perform with devastating results. To wit:
Dead-angling, where you circumvent an opponent who has their shield up with an attack which is likely to hit their unprotected sides without even trying. Of the better weapons to abuse this somewhat questionable strategy, none are more dangerous than the Great Scythe, which also causes bleed damage to keep the pressure on your unfortunate victim, the Murakumo, whose moveset is almost entirely based on this strategy, and the Black Knight Greataxe, which was already explained above.
Turtling, in which you use a spear or rapier and hide behind your own shield (heavens forbid you if you're using Artorias' Greatshield) to poke your opponent to death. This strategy got slightly nerfed in a patch which increased the stamina drain when doing thrusting attacks with your shield up, but it's still annoying to face one since you have to risk trying to break their guard while taking damage from their pokes.
Backstab fishing, where the player does nothing but circle around their opponent in order to backstab them, allowing for instant massive damage from a Critical Hit, made even worse if they use the Hornet Ring for a solid 33% (50% pre-patch) increase in critical hit damage.
All the above are not so bad compared to stacking Poise and using any weapon (usually a Game Breaker like the Claymore) that can easily stunlock low-poise opponents, a strategy otherwise known as poise race, where the fight usually degenerates into which player can break the other's poise first and stunlock them into oblivion.
Before a patch that reduced its overall usefulness, the Iron Flesh Pyromancy was known to turn boss battles into a cakewalk due to the massive increase in physical defence and poise, with the added effect of making light and medium attacks bounce off the user. And since it's a simple passive effect without any stat adjustment, you don't need a high Intelligence stat or a maxed-out pyromancy flame to enjoy it.
Because pyromancy doesn't scale with any stats other than Attunement slots, it's very common for low-level griefers to bring in a fully upgraded pyromancy flame and a powerful spell like Black Flame or Fire Tempest, allowing them to wreak havoc against equally low-level players. Most beginners have a hard time countering this sudden leap in power, let alone have the reflexes to dodge them if it's their first time.