Fable II

Easiest way to get this by far is to use a stage three time slowing will power (forget the actual name of it) because not only does it slow your enemies but it slows the clock as well. Then just cooked everyone with a stage 4,5 inferno, rinse and repeat.

Using this method I got to the necromancer with well over a minute left on the clock. Then you can just continue to use the time slowing will and pop him in the head a few times using the sub-targeting ability.

Have 5 star time stop and 5 star thunder or fire. Use two controllers, one for time stop, one for thunder or fire. Thumbs on both B buttons, and just keep unleashing full charge attacks. I had almost three minutes on the clock when I got it.

Simple solution, do NOT worry about hitting the orbs for multipliers or kicking the chickens. I found the best way is to just use melee attacks, don't waste time charging magic / aiming with your ranged weapon. You need all of the time you can get.

And keep in mind that sometimes one remaining person likes to sit on the other side of the arena then where your are, so if everyone is dead and nothing else spawned, kill him quickly.

Every second counts for this one.

HENCHMAN RECOMMENDED FOR THIS!

keep fighting in the arena untill you run into spire guards... try to have at least 1 min left when the entire field is full of them then just keep fighting till none are left

the easiest way to kill a necromancer is to shoot him in a somewhat close area to him, he will teleport but your hero will track him after a few dead hollowmen

Here is an easier way to use KillKarZd's strategy

Get two players
One stops time, the other attacks
When the necromancer comes stop time and double team him

The Daichi and The Rammer are highly recommended!!!

I got this first attempt by only using X attack and not using Y or B because it takes far too long.
Forget the multipliers, they're not necessary for this specific acheivement. Try to be as quick as possible when attacking and if they block attack a different target, you will end up wasting time charging your X attack.

When the Necromancer appears shoot him in the Head with the Rammer by using sub Targeting and it should kill him in one/two hits. If he spawns any hollowmen try to get past them as quickly as you can by using the Daichi.

Video Guides for weapons below.

I managed to do this with just a Melee weapon and single-target Time Slow.

Buy the highest damage weapon you can find. Mine was an axe with 107 damage and 3 Augment slots. I purchased it from the Stonecutter on Knothole Island. Buy any combination of damage and armour Augments. I used an increased damage, increased armour and increased damage against Evil enemies. Ranged weapons are not relevant.

Before starting the arena, I full upgraded all of my Strength and Skill abilities and fully upgraded Time Slow.

When you get in the arena, you want to spam X as much as humanly possible. Focus on one enemy and move onto the next. Single target Time Slow is great for getting enemies that are sitting on the outer rim of the circle. If you come across an enemy that tries to block, instantly move onto another one. Also, barrel-rolling is your friend. Moving between enemies with rolls is a fast way to pick them off while also avoiding damage. With my weapons and augments, I was 2 hitting most enemies, including Red Balverines.

DO NOT USE RANGED WEAPONS

It takes too much time and leaves you open for Balverine combos and getting knocked back from ranged attacks.