How do I use command blocks to make a mob arena in 1.8?

Solution 1:

Yes, you can do it as they say, but you can automate it.

You will need some redstone mechanism to delay spawning or actually start it (button). That is on you, how fast commandblocks got executed (you can use buttons for every wave or redstone clock or repeaters or something even slower - two hoppers with comparators or so. You need to know something about redstone for this.

Second, you need command blocks. Alternative is to use dispensers with spawn eggs, but commandblocks are more fancy.

How to actually setup them? Yuu can use this page: http://www.minecraftupdates.com/summon-command

You can setup with them spawners - for constant spawning (for stopping it you need commandblock to delete the spawner) If oyu want to start it by pushing button, then the button has to trigger CBs that generate the spawners.

Or you can do /summon command for summoning one monster at time. Then you need more command blocks, so it will be more redstone size. However you can select the place where the monster will be summoned etc.

The positive of mentioned page is that you can define more values in dropdown menus, armors, potion effects and so. Also traps like primed TNT are possible. Thus, your arena can offer really challenging fight, like skeletons in diamond armors or invisible blazes!

Solution 2:

You have several options:

• Summon a Zombie Spawner. Using commands, you can summon a spawner of a certain mob type. This can be achieved for any and all mobs in Minecraft - both passive and aggressive mobs. Eg: /give @p minecraft:mob_spawner 1 0 {EntityId:Zombie}^[1] will summon a zombie spawner.
• Change the spawner type. In creative mode, you can now change the spawner type simply by using (right-click) an egg on it. Again, this works with all mobs.
• Use a Redstone Clock. If you want to use command blocks, this command will occur every time you input a Redstone Signal. So you can either sit there and punch a button every 3 seconds, or use a Redstone Clock. This will emit a Redstone Signal automatically, so long as it has an input. This means you can turn the system on - automatic spawning; turn it off - no more spawning.

^[1] Syntax needs citation