As Olorun, reach the maximum critical hit chance possible with Touch of Fate.
Touch of Fate is Olorun's passive ability. As you build magical power, your critical hit change increases as well. Upon reaching 150 Magical Power from items, you will gain 15% Critical Hit Chance. From there, you'll gain a further 1% per 10 Magical Power from items. The maximum critical hit chance is 70%. This achievement is simply building items with high magical power. I built Bancroft's Talon, Spear of Desolation, Telkhines Ring, Rod of Tahuti, Chronos' Pendant and Doom Orb. This is not a good build (and is very expensive) but should get you the 70% required. This build is best attempted in Arena.
I just hit 70% again, in Arena, with an alternative build (also went 22-11-16 in this match): Demonic Grip, Bancroft's Talon, Telkhines Ring, Rod of Tahuti, Shaman's Ring, Spear of Desolation (and then an elixir of speed).
When you reach around 60%, make sure you wander into combat and allow yourself to take some damage, as the effect of Bancroft’s will increase your crit percentage.
You could also attempt this in a longer game mode, such as Conquest, as you’ll almost certainly amass enough gold to make a full build and switch out items where necessary. A better build would for use in conquest would include Hunter’s Blessing (later sold for an optional item, such as Book of Thoth), Bancroft’s Talon, Shoes of the Magi, Demonic Grip, Telkhines Ring and Rod of Tahuti.
Remember, in both modes (especially conquest) once you reach a full build with boots, you can sell your boots and buy for an Elixir of Speed. This frees up another item slot where you can pick up another power item.