It is very simple. MF is a champion who, when she builds from lethality, has a huge Burst potential with her damage (especially her R). Her ability to hit several targets, in addition to being decisive in teamfight multiplies the amount of Gold generated thanks to First Strike. The trick is to wait for the rune to be ready and be ready to use your best ultimate. You can generate up to 500 gold on one ultimate on 5 people with only 3 items and that's without counting the kills/assists you get from your action. In lane, you will have to privilege short trades in the proke window of the rune.
Axiom Arc is crucial in this build since it rewards you even more pixel ultimates. The more you use your ult correctly, the more often you have it and the worse you will do because the more gold you generate. The scaling of this build is simply insane.