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.