Forcing mobs to target players in MobArena regardless of range

You can set the follow range of mobs higher. Assuming you want to summon a zombie:

/summon zombie ~ ~ ~ {Attributes:[{Name:"generic.followRange",Base:100.0}]}

This zombie will follow you up to 100 blocks far.

(You can also let mobs directly target specific players or entities, but for that you would need to know their UUID, so it would only ever work for one player in the world.)

If you are spawning in the mobs individually, then as Fabian said:

Assuming you want to summon a zombie:

/summon zombie ~ ~ ~ {Attributes:[{Name:"generic.followRange",Base:100.0}]}

This zombie will follow you up to 100 blocks far.

However if you want to customise this for every mob, especially if used in a plugin like MobArena, do this:

/entitydata @e[type=Zombie,r=150] {Attributes:[{Name:generic.followRange,Base:150}]}

Change Zombie to whatever mobs you want