You didn't need mods for this! Just follow the steps
1. Open your Minecraft world/ Connect to the server you want to install into it.
2. Give yourself a command block by /give @p command_block in the chat. You need to allow your cheats, if they are not allowed, just click "esc" >>> Open to LAN >>> Allow Cheats:On >>> Start Lan World and you are ready! Also if you are on server, you need to be opped!
3. Paste the command from the link above into the command block and power it with button or redstone block/torch
4. Enjoy!

How you read above, the Ender Dragon and the Wither has more abilities and can kill you easly. I know, that are little boring these abilites, but they makes the bosses more difficult, so don't be chicky :D. Let's see the abilites:
One Hint: The abilities are random executed, so you can't expect what it will gonna be...

Ability
"What it says"
What makes


Ender Dragon's Abilities:

Poison Gas
"Poison gas, Go!"
It gives you poison... painful poison

Tnt Bomb
"Look up! Be Blowed!"
It spawns a Tnt above you, that it has a big Fuse and it will blow after few seconds, when it is on the ground

Shulker's Ability
"Ah... The levitation is good thing!"
It gives you levitation, of course! :D

Single Arrow and Lava
"A single arrow? Really... You are stupid!"
It spawns arrow above you.. and a block of lava, so be careful to not be killed by a stupid lava...

Fired Floor
"Be fired up! Bluaahrs *Growl*"
It filles the area around you with fire, that causes to be.. on fire?

Freeze!
"Freeze! NOW!~"
It gives you slowness, that you can't move from it

Teleporting to the Beast
"I... will... eat... you!"
It teleports you to the dragon, so...

=================================================================================

Wither's Abilities:

Lightning Bolt
"Lightnings! Beat him!"
It spawns lightning to your location

Ghosts
"Dear ghosts.... EAT HIS BRAVENESS MUHAHAHAHAHA"
It spawns four "Ghosts" near you (from Ghosts I mean invisible charged creepers, just be careful..)

Cold Weather
"Cold weather! One more trapped idiot!"
It cages you into packed ice

Blind Power
"Some gun powder in your eyes, and you will be done!"
It gives blindness, and it will be very hard to see...

Multi-Bomb
"Arrgh! Multi Bomb!"
It spawns a Tnt, that rides a Tnt, that rides a Tnt (3 Tnt's at one location...)

Poisoness Paralyze
"Paralyze Poison! GOOO!"
It gives you slowness, that you can't move and poison at one time

One More Wither
"Hey, my twin! Boom him!"
It spawns one more Wither, but with Tnt, so it will blow...

=================================================================================

Now I will start with the Magma Pyromancer's abilities, later with the recipies you need for him.

Magma Pyromancer's Abilities:

Lava Floor
"Die in lava!"
It fills the floor near you with lava.

Nether Dragon Breathe
"Nether Dragon Breathe! I... Will... Kill... You!"
It gives you wither poison and a cool particle... It is like Ender Dragon's Breathe

Poisoness Levitation
"Enjoy the Air Poison... Muhahahahaha!"
It gives you Levitation and Poison and a big falling damage... yeah

Magma Spair's Arena
"Magma Spair! Kill him, or you will die!!"
It fills the floor under you with air (you will fall in hole) and it spawns another Lava Slime

===

They are abilities of Magma Pyromancer, that for certain time are executed

Magma Frags
"Magma Frags! Kill him, NOW!"
It spawns several Endermites, that chases you and wants to kill you

Slime Minions
"Slime minions, don't die, or you will go to the nether and live there all your life!!"
It spawns Slimes, that chase you too :D

Lightnings
"Let the lightning bolts eat you braveness!"
It spawns several lightnings at your location

=================================================================================

Now let's see how to make the Catalyst Box and the other things, you need to know.

To make the Catalyst box, throw at one location an obsidian, an beacon and iron block. It will gives you two Catalyst Boxes, because when you destroy a Catalyst Box, it won't drop it.
When you are near the Catalyst Box, it gives you Fire Resistance, Strenght and Regeneration - suitable for fighting with the upgraded bosses. Also I prefer to fight with the bosses, when the Catalyst Box is near.

To destroy the Catalyst Box, throw at it Obsidian. It costs nothing, because it will not delete the obsidian block.

To make the Magma Pyromancer's egg, at the Catalyst Box throw a redstone block and a nether star (first you need to kill the Wither for that, so...)

You need to know too: When you hit 80 times the Magma Pyromancer you will kill him, and also when you kill him, you will have an ability, that of radius 50 blocks you will kill EVERY lava slime!

Also it will gives you an item "Magma Pyromancer's Heart", that when you drop it, it will summon Lightnings of every mobs near it, or it will gives wither, or it will makes more things like this, or will do nothing.