This blog is to teach you how to fight the hostile mobs of MC safely so you won't die. Super helpful of hardcore!!

Creeper-
Use a bow to kill them from a distance. You can also hit them with a sword and quickly run back to avoid an explosion. You don't want an explosion, as it can grief your structures and leave a hole in the ground. When fighting a charged creeper, make sure to be EXTRA careful, as they have a bigger explosion that can easily kill you. Note- Creepers can be used to damage multiple mobs by making them detonate next to other hostiles, as long as it's before 1.8!

Skeleton-
Use a bow and blocks to take cover behind. Try to have them shoot another skeleton or zombie, so they will attack the origin skeleton, distracting him, weakening him, and maybe even killing him! Be warned, fighting more than one at a time may result in death, or broken armor. When fighting a skeleton, don't focus on him too much, as a creeper may come up behind you.
HINT: Having him shoot a creeper to death drops a music disc.

Zombie-
Things you should know... they can spawn other zombies around them to help, they get increased attack power when they have lower health, can see you and villagers from far away, can set you on fire when they are on fire, and other dangerous tricks, such as wooden door breaking on hard mode. Have good armor, and a good sword that won't break. Be sure to have your sound on to hear their noise and always watch you back. For Baby Zombies, know that they don't burn in the daylight, and are pretty fast. But they have the same hit box as a 2 block tall mob, so aim for the block above their head.

Spider/Cave Spider
Both spiders are fast, can climb walls and will jump to attack. Cave spiders can also poison you on normal difficulty and above. You should use good armor to take them head on, or use a bow. If using a sword, be sure to run back to avoid damage, as they can easily overpower you. Both spiders are hostile in a lower light levels/night, and are neutral in higher light levels/daytime.

Silverfish-
While they can be easily be killed, they are equally dangerous. They spawn from spawners in stronghold's portal room, and rarely through monster eggs in the extreme hills biomes. Also, if you attack a silverfish within a certain distance from monster eggs, they can break and more silverfish will attack you, resulting in a swarm. Use a sword to kill them fast, but we warned of the above info and this quote.
Monster eggs come in stone, cobblestone, stone bricks, mossy stone bricks, cracked stone bricks, and chiseled stone bricks.

Witch-
They can heal themselves, as well as poison, damage, and slow you via potions. When using a bow, be sure to use charged shots and possibly use enchants. With a sword, rapidly click the mouse to hit them very fast to kill them very fast. As of 1.7 they can spawn at night, not just in their huts, so be very careful.

Slime/Magma Cube
Both have multiple stages in different sizes. Killing the big slime makes small slimes, killing one of those makes tiny slimes(numbers vary). Same applies to magma cubes. Armor is a good thing to have, since they can easily grow in numbers. Magma cubes are even more dangerous, as they may push you off the edge of a platform, landing you in lava or killing you from fall damage. They can both see through walls.

Blaze-
Blazes are very dangerous. They are spawned in nether forts via spawners/inside. One of them is very dangerous, and the spawner may spawn multiple. Snowballs can hurt blazes, so use them along with bows. A fire resistance potion on you will render them nearly harmless to you, as the fire won't affect you. The can still hit you if you get very close, but you won't need to, as ranged weapons are always best against them.

Ghast-
Use a bow and have plenty of arrows. Their fireballs can be deflected away from you and even back at them. So shoot it back or hit it back at them. Make sure they are by fire-less land if you want their drops. Hint: Fire doesn't hurt them. Flame and fire aspect won't do any extra damage.[Yes, swords can be used, but are pretty hard to use against Ghasts] Cobblestone should be used against anti-Ghast forts / cover, as they can blow up most other sources, and cobble is very easy to obtain.

Wither Skeleton-
He is pretty powerful with his sword, and can leave you with the wither effect. With this in mind, ranged weapons are better to use against him. If you are fighting him melee, you should bring a milk bucket or two. They rarely drop wither skeleton skulls, used for the Wither, so don't knock them off platforms to places you can't get the drops, just in case he drops a skull.

The Killer Bunny-
A very rare version of the Bunny, from 1.8. It is out to kill you, with up to 12 hitpoints of damage![On Hard +] To counter him, have good gear. If you encounter him during your beginning of the map, just run away and lose him, as he can easily overpower you. It attacks wolves as well, whether wild or tamed. They also don't despawn on Peaceful, so don't try and cheat :P Instead, hit it back and run away fast. Try to cross a river or parkour out of it's range. Keep in mind that they are much faster than the average rabbit.

Guardian-
The new 1.8 underwater mob, made to protect the new Water Temples. These guys fight using eye beams that charge up before hurting you. They swim pretty fast underwater, so you are at a disadvantage. Their spikes also damage you if you come in contact with them. While out of water, they will hop around, probably hitting you and hurting you. Bows can be your life saver here, as you won't have to worry about the spikes. Be sure not to get attacked by multiple Guardians. Since they only spawn around/in the temples, you won't have to worry much about them. They also hunt squid, so feel free to take the dropped ink sacs you find.

Elder Guardian-
Unlike the regular Guardian, the Elder only spawns in one of 3 locations inside the temple. 3 spawn for every temple. The Elder has a bigger health bar, along with bigger hitbox and power. Hiding behind objects is essential. Enchantments will help you get the job done faster. If possible, draining a small area of the temple will give you a hiding hole to fall back to. Note, they will give you the potion effect "Mining Fatigue III", which slows down your arm swing. This effect takes place in the Temple's area, so try and take out the Elders before doing mining inside the monument.

Ender Dragon-
Once you are inside The End, the dragon will try to kill you by flying straight at you. Full diamond gear needed + Bow with Infinity / many, many arrows. Make sure to avoid edges, and avoid looking at the Enderman. Armor enchantments encouraged while fighting it.
While fighting the Dragon, he will be healed by the Ender Crystals mounted on the Obsidian Pillars. Use your bow and shoot them to destroy them, or bring along snowballs and throw them into the Crystals, blowing them up as well. Blow them up while the Dragon flies by to cause blast damage to him. Be sure to get all of them, or he will continue to regen health.
A pumpkin can be used to avoid Enderman hostility, at the cost of armor points. A resource pack can be used to make the pumpkin texture 'view friendly', though it may be counted as cheating. Water can also be used as a anti-Enderman method, but not encouraged.
Keep in mind that the Ender Dragon can break all blocks, except for Endstone, Bedrock, and Obsidian.

Diamond and sub :)