Information on Monsters
When you move the cursor over a monster, you see the name of its class. Behind the name in red appears the monster's hit-point bar graph. It shows the percentage of hit-points remaining. The name of the monster is color-coded. White is a standard monster type, blue is a champion, and gold is a unique monster.

Monster Difficulty and Experience in Multiplayer games
Monsters' experience value and Hit Points will change with the number of players in the game. When more people join a game, the monsters become harder, and as they leave the game, the monsters become easier. For the ultimate challenge, fight solo against monsters on Hell difficulty in an 8-player game. In multiplayer, when a monster is created, its Hit Points are multiplied by the number of players in the game: (Life = Hit Points * (Number of Players + 1) / 2). The monster's base experience value is determined by the following formula:

Experience = X * (n + 1) / 2

X = base experience
n = # of players in the game

Typing "/players X", where X is a number between the current number of players and the maximum (8), in the message box will set the effective number of players in the game (in single player, open Battle.net and TCP/IP games only).

Monster Level vs Player Level
Monsters that are a higher level than your Character have an easier time hitting you and you will have a harder time hitting them. You will have an easier time hitting monsters that are a lower level than you, and they will have a harder time hitting you.

Monsters Spells
Monster spells always hit as do magic (non-arrow/non-javelin) ranged attacks.

Can you list Monsters Spells, Spell Levels, and Damage?
Details on the damages are very hard to get so we will probably not be listing this information.

Monster Type
Monsters are either Demons, Undead or Animals. Any monster that doesn't say Demon or Undead is considered Animal.

Immunities
Some monsters have Immunities to Magic, Physical Damage, Poison Damage and Elemental Damage. This means they have 100% Resistance. A few monsters in Normal Difficulty have Immunities. In Nightmare Difficulty, more monsters have Immunities. In Hell Difficulty almost all Monsters are immune to something.

How can I damage Physical Immune Monsters as a Melee Character?
Melee characters need to use Magical or Elemental Skills or Weapons with Elemental (Fire/Lightning/Cold) or Poison Damage. Paladins can use Vengeance and Barbarians can use Berserk. You can also use your Hirelings such as Act III Hirelings to help out or rely on other party members. Elemental Damage Charms also work very well. Collect high damage Elemental Charms.

Monster Health Regeneration
On higher difficulty levels, Monsters regenerate health very quickly. Start a Hell Difficulty game and attack a monster until he's almost dead. Stop attacking him and place your mouse over the Monster to see his Health Bar. Notice his health quickly regenerates if you don't attack him. You can use the magic bonus "Prevent Monster Heal" on weapons to stop monsters from regenerating. It's important to finish monsters off, and to stay on the same target (hold down the mouse button on a monster), so that monsters do not regenerate back to full health while you're not attacking them.

Monsters on Nightmare Difficulty
On Nightmare Difficulty, Monster Level, hit points, Armor Class, Experience, Damage, Chance to Hit, and Resistances all go up.

Monster skill levels go up +3.

Unique Monsters gain one bonus ability.

Monsters on Hell Difficulty
On Hell Difficulty, Monster Level, hit points, Armor Class, Experience, Damage, Chance to Hit, and Resistances all go up.

Monster skill levels go up +7 in hell.

Unique Monsters gain two bonus abilities.

Try to avoid taking on large groups of monsters at once. Try to pick away at them until there is nothing left by hitting and running, whenever possible.

When facing tough Unique monsters which will result in a long fight, clear out all of the Unique's Minions first, then focus on fighting the Unique monster.

Always attack monsters that revive other monsters first. Rush through a crowd of monsters to the monster capable of raising and quickly kill them.

Cold is very helpful to prevent monsters from being Revived. Monsters that are Chilled have the chance of being shattered upon their death which will leave no corpse to be raised. Monsters that are Frozen will always shatter when killed. Group Freezing spells can also quickly stop a huge army of monsters and break them up, making them easier to kill.

Another trick for dealing with monsters that raise other monsters is to lead the minions or monsters around them away from the raising monster, which will generally stay in the same area. Once you've lead the minions enough distance away, kill them then go back to finish off the boss monster.

Some monsters have powerful ranged attacks but weak melee attacks. If you rush up next to them, they will use their melee attack which may be far less damaging than the ranged attack they were using before.

When in trouble, run. Run to another area if possible using dungeon doors, stairs, way points, and town portals to get away from the monsters.

Melee characters should avoid being surrounded. Fight for a bit, then back away, to keep the monsters separated. All characters will find this useful for controlling the amount of monsters attacking you at one time. Don't be afraid to hit and run until you can reduce the number of monsters.

Use Bridges, Doorways, and Narrow Passages to reduce the number of monsters that can attack you at once.