Darksiders

This will involve a few rounds of arena fighting.
The you will face a boss who is on top of ruin.

Circle him till he charge, dash to the side twice, then follow up with a few hit.
After doing this a few times you will get the option to B on him, thus dismounting him.

When he fires the blue homing ground lighting just time and dash to avoid.
Got up to him and swing 3-4 times, if he swings at you dash, back twice, or even better if you have bought the ability, jump up to dodge the first swing then air dash backwards to avoid the second.
After a while he will throw 4xblue homing ground lighting just dodge left or right at the last moment.
If he jumps in the air, just dash to the side, then dash backwards.
Repeat until dead.

Then its time to fight just the horse, hit it a few times, dodge its attacks.
After a little while a cut scene will come in, after the cutscene, you can now ride Ruin and you will unlock this achievement.

This is story related and cannot be missed. When War obtains his horse, Ruin, the achievement will unlock.

The following guide assumes the player is playing on Apocalyptic mode but is also applicable to lesser degrees of difficulty:

As War makes his way through Ashlands, he will end up in The Gladiator Arena. There, he will face four challenges. There are two health chests. However, the challenges get increasingly more difficult so War must pace himself.

First, War will be challenged by a single enemy on horseback. After War depletes its health, the horse rider will summon reinforcements. Upon defeating the horse rider and the reinforcements, a waypoint is established and two more horse riders with a couple of minions will show up as the second challenge.

The single horse rider with and without minions is a good introduction to the more challenging battle with two horse riders. The horses can inflict severe damage. They telegraph their attacks by rearing up. They are more vulnerable from the back and sides. However, they have a damaging rear kick so side attacks are the least risky.

The horse riders can be attacked front, side, & rear. They telegraph their attacks so War can attack and dash escape with . Needless to say, Mercy or Crossblade ranged attacks can augment the close-in melee attacks. It is best to stay on the move alternating attacks with evasion. The Blade Geyser wrath attack is effective against them. Should this attack be used, it is best employed when War and the two horse riders are close together so a single Blade Geyser attack will hit both horses.

The third challenge consists of two Traumas with minions. The first Trauma engages as soon as the third wave starts and, shortly after, is joined by the second. The Traumas are slow so evading with is a good tactic, hitting enemies with ranged or melee attacks between evasions.

When all enemies in the third wave are defeated, a way point is established and the final enemy appears--the Abyssal Gladiator. This is a mid-boss battle but the Abyssal Gladiator is vulnerable to War's chaos form. I recommend not using the chaos form immediately. The reason to wait is because the Abyssal Gladiator is less formidable on horseback when he first appears than on foot after he has been unseated.

This guide's recommended tactics are to unseat the Abyssal Gladiator using the same approach used to fight the Abyssal horse riders. After the Abyssal Gladiator's health has been depleted a prompt will appear. Pressing will unseat the Abyssal Gladiator. Following a cut scene, War will fight the Abyssal Gladiator on foot.

Assuming the Chaos gauge is full, activate War's chaos form. Attack repeatedly with . The Abyssal Gladiator will succumb and War will be attacked by the horse, Ruin. Continue, while in chaos form, to attack with . If War's chaos form expires before the cut scene starts, evade with dashes. Assuming you were efficient with War's chaos form attacks, a cut scene will begin. War will obtain the horse, Ruin, and the achievement will unlock.

Without using the chaos form (Note that some or all of this paragraph may be applicable in case of inefficient use of War's chaos form): The Abyssal Gladiator is a dangerous opponent. Staying on the move is essential because, in addition to devastating melee attacks, he has homing range attacks. Nevertheless, the strike (melee & ranged) and move tactics recommended throughout this guide continue to be effective. When the Abyssal Gladiator is defeated, the horse, Ruin, will attack. At first, War must counter Ruin's attacks and strike back. As events unfold, it becomes apparent that War must evade with dashes. After a short interval evading, a cut scene will begin. War will obtain the horse, Ruin, and the achievement will unlock.