The necromancer

This minecart-headed thing with the fire particles is something called the Necromancer, a custom boss. Instead of arrows, he shoots ghast fireballs, and he spawns legions of undead minions with enchanted diamond swords. Beat him, and you will be rewarded with a lot of experience. He has no special drops yet.
No, he doesn't look the neatest. But he gets the job done.

One way to make an instantaneous bomb in minecraft

This is a bomb that explodes instantly. You step on the pressure plate, triggering the dispenser to fire a projectile at the Ender Crystal, which explodes.

How to make a Laser Tower in vanilla minecraft

A few steps to make a working laser tower that fires at players.
Start with an iron block, then place two iron bars, followed by a command block.
Paste this command into the command block:
/summon Guardian ~ ~2 ~ {Invulnerable:1,PersistenceRequired:1,Silent:1,ActiveEffects:[{Id:14,Amplifier:1,Duration:1000000,ShowParticles:0b}],Riding:{id:"Silverfish",Invulnerable:1,PersistenceRequired:1,NoAI:1,Silent:1,ActiveEffects:[{Id:14,Amplifier:1,Duration:1000000,ShowParticles:0b}]}}

Activate it, then destroy the button and replace it with iron.
As a final touch to the decor, summon an ender crystal.
Then tweak it with the tp command until everything is in the right place.
Switch to survival and it should fire at you and other players.