The bosses are the Three Prime Evils, the lesser evils, and their lieutenants and underlings. They have special abilities, attacks, appearances, and are without a doubt the toughest monsters you will find in the game.

Andariel:
The final quest of Act One will be killing Andariel, the Maiden of Anguish. One of four lesser members of the Seven Great Evils, she will be surrounded by some minions.

Andariel has a deadly poison attack she will cast when you get up close. Whenever possible fight from a distance. Open up a Town Portal before fighting Andariel so that you can easily get back to your body if you die. You can also use this Town Portal to escape if the fight goes badly. You can lead Andariel in circles around the little lake before the two doors into her lair if you need time to heal or for Healing Potions to take effect.

Duriel:
The Tomb of Tal Rasha holds many surprises for the unwary, and this demon is the biggest surprise of them all. Though Deckard Cain gives you a few warnings regarding the troublesome issue of Lesser Evils collaborating with the three Prime Evils, you still have little idea of the boss you'd be facing as you stepped into Tal Rasha's chamber. At this point, you still believe your enemy would be the Lord of Destruction himself. Duriel's first taunt as you enter the chamber, "Looking for Baal?" is evidence enough that your prey eluded you. Baal and Diablo, having defeated Tyrael, left and sealed up the chamber with the archangel still trapped within. For good measure, they placed their younger brother Duriel to guard the tomb.

Duriel's Special attacks are a Holy Freeze like Aura, Jab, Smite, and Charge

Duriel uses an Aura similar to the Paladin's Holy Freeze. While the Aura appears to be a Cold Damage Aura, Cold Damage Resistances and "Cannot Be Frozen" will offer no protection from Duriel's Aura.

Open up a Town Portal inside Duriel's lair or just outside so that you can get back inside if you die.

Sorceress: Use Static Field to bring Duriel's Hit Points down, then finish him off with another spell. Necromancers use Minions to focus Duriel's attention then cast Iron Maiden or use Bone Spear, Bone Spirit or Teeth. Amazons use Valkyries if possible to draw Duriel's fire. All classes can bring a Hireling to keep Duriel busy for a few seconds. You may wish to move to melee range which will prevent Duriel from using his Charge Attack.

Mephisto:
Mephisto was the first Prime Evil to be captured and his stone was buried in a vault beneath the Aranoch desert. Mephisto finally managed to overcome the powers of the Soulstone that imprisoned him enough to influence and corrupt the local priesthood, known as the Zakarum.

Clear out any nearby Council Members or monsters in the area. If you're unable to kill some of the Council Members you can use this trick: open up a Town Portal in the middle room with Mephisto or the left room. Then lead any Council Members you're unable to kill to the stairs that lead to Durance of Hate Level 2. Now run back to the Waypoint in Durance of Hate Level 2 and use it to go back to Town. From Town enter your Town Portal and you'll be back in Mephisto's Lair. The Council Members will be back at the stairs out of the way, if you lead them away correctly and placed the Town Portal far enough away from the stairs. You'll now be able to attack Mephisto without his being helped by those council Members.

Cold and Lightning Resistance helps a lot against Mephisto.

Mephisto uses the following Skills: Lightning, Charged Bolt, Poison Nova, "Skull" Missile, Frost Nova, Blizzard.

Diablo:
Diablo spent two centuries slowly working to corrupt the Soulstone that imprisoned him. In time, he was able to extend his influence into the surrounding area and corrupt both King Leoric and his archbishop, Lazarus. The King proved too strong to fully control, so the Demon took possession of his son, Prince Albrecht. Diablo then began to shape an outpost of Hell within the catacombs that ran beneath the town of Tristram. By spreading terror into the surrounding countryside, the Demon was able to attract many heroes who came to cleanse the land of evil. By the time the strongest of these heroes reached this goal, though, he had become fully influenced by the will of Diablo. In his twisted state, this adventurer believed that the only way to fully control the Demon was to plunge the shard of the Soulstone into his own head. This, of course, was exactly what Diablo had planned as the Demon now had an even stronger body to use to find his brothers and complete his ultimate plan

Full Lightning and Fire Resistance will help a lot against Diablo. With high Physical Damage Reduction and Fire/Lightning Absorption, Diablo will end up healing you over time instead of damaging you.

Diablo may cast his special Bone Prison on you, your hirelings, and around your Town Portals to prevent you from escaping.

The Red Lightning Hose deals half physical, half lightning damage. Full Lightning Resistance and Damage Reduction/a high Block Rate may allow you to survive this attack.

You cannot steal Life or Mana from Diablo.

Diablo uses the following Skills: Fire Nova, Red Lightning Hose, Cold Touch, Fire Wall, Firestorm, Bone Prison, Charge, Taunt (does nothing).

Baal:
Years ago, Baal was captured by the Horadrim Order within a soulstone. In the Act II, the goal of the player is to locate the tomb of Tal Rasha, and to prevent Diablo from getting there first to free Baal.

With Diablo's death and the destruction of the soul stones the Archangel Tyrael, informs you of another problem and asks you to follow the path of Baal, the last of the Prime Evils, into the Barbarian Highlands of the north. Baal intends to corrupt the powerful Worldstone, which protects the whole of the mortal plane from the forces of Hell.

Rush Baal and quickly do some damage to him. Don't waste time attacking the "fake" Baal. The best way to beat Baal is to just rush him. Static Field and Crushing Blow is very nice for bringing his health down. Hirelings can also be very helpful, especially the Act II and Act V hirelings.

High Physical and Cold Resistance helps a lot against Baal. His Hoarfrost attack can shove you away from him, giving him time to spawn a Vile Effigy or follow up with Mana Rift. Dodge the Hoarfrost whenever possible and try to corner him, preferably in the tight space near the Worldstone where he can't maneuver. Bring Rejuvenation Potions to prepare for his Mana Rift, which can be very damaging to all classes. If he curses you with Blood Mana you may want to throw up a Town Portal to have it removed. Should he unleash Festering Appendages on you, relocate yourself around Baal so they can't get free hits.

Though there are stacks of gold that litter the Worldstone Chamber, it's best not to pick them up until after the fight is over.

Baal has many special attacks:

Teleport
Baal can Teleport around but he cannot heal himself in the process.

Decrepify
This curse is the same as the Necromancer's Curse.

Defense Curse
Baal can cast a curse resembling three converging needles which cuts your Defense by 60% in Normal, 75% in Nightmare, and 95% in Hell.

Blood Mana
Succubi and Baal cast a special curse shown in the screenshot below (Along with Thunderstorm and Energy Shield). This curse causes you to do damage to yourself when you cast spells. For skills that require more mana, you will take more damage. This even includes such spells like Teleport but doesn't include Thunderstorm or Energy Shield because you have already cast them. Whenever this curse is cast on you, stop casting spells until it's worn off, click on a Shrine to counteract it, or go back to town and have it removed. Blood Mana only affects you if you have more Mana than Life. If you have at least as much max Mana as max Life, you can be cursed.

ncineration Nova
Created from fires summoned forth from the underworld, this explosion of hellfire singes even the most stalwart warrior of light. All enemies of the Lord of Destruction are bathed in this destructive force, leaving only ashen remains in its wake.

Hoarfrost
The cold embrace of the grave seeks out those foolish enough to invade Baal’s inner sanctum. This icy wedge drives itself into its targets, pushing them back while slowing their movements as they rapidly freeze to death.

Mana Rift
The power behind magic is an elusive force to master and can be fleeting, as well. Rending the very fabric of these forces, this spell eradicates half of the mana the victim has stored, leaving him sorely underpowered for the battle to come.

Destructive Strike
Lashing out with unholy strength and fury, Baal shows that his attacks are not limited to the arcane arts. Able to hold his own against any foe in melee combat, his touch can tear down even the strongest defenses. In Nightmare and Hell, Baal's Destructive Strike can be enhanced with Fire or Cold damage.

Festering Appendage
There is no escape from the Lord of Destruction, for even those that choose to run are confronted with his far-reaching grasp. Standing in the way of any escape are these resilient limbs, impeding the backward progress of his adversaries.

Vile Effigy
Should Baal be confronted with overwhelming numbers, he can tip the odds back to his favor by creating an exact duplicate of himself. Having every power that his creator possesses, this replica doubles the damage and destruction that Baal can create. The clone is created with the same percentage of life as Baal. When damaging the clone, a player will see the clone's Health Bar drop faster than the real Baal.