A slight introduction to MF:
MF; In my opinion is the most well rounded burst mage in the game. I genuinely wouldn't call her an ADC As her entire kit is rounded around three things. One Auto. One Q. One Ult.
Unlike other adc's, MF Doesn't have the ability to Kite around to do Consistent damage.
Your entire game is based around your burst.

Positioning is just as Crucial on MF As it is any other ADC In the game.
Poor placement in a fight can get you one shot or perma cc'd
MF'S Ultimate is the most important part of your team fight and can shred entire teams. Wait for a nice initiation from your team or create your own to burst the entire team for easy kills and wins.

The Key to this build is a strong late game. Dark Harvest is the single most powerful rune on MF. Stack into the late game and build up to easy One shots with a Q Crit.

Taste of blood works incredibly well with Mf. Generally compared to Fleet footwork excluding the Movement speed.

One of the weaker runes in the game, but powerful none the less. Adding extra ad to your late game; it perfectly compliments the Absolute Focus and Gathering Storm Combo

Stacks incredibly well with MF's passive Strut . Providing a total of 40 movement speed directly from the Rune.

Absolute Focus provides a total of 24 Extra AD while above 70% hp.

Games are usually decided and won at 30 Minutes into the game. BY This point gathering storm is already providing a whopping 29 AD.

MF Has an incredibly early game. Her base damage early on helps her to come out on top in trades consistently and farm efficiently.

MF's W "Strut" Provides a total of a 100% Movement speed boost at level 5. Helping her bob and weave to dodge effectively anything.

MF's E "Make it Rain" Provides early brush vision to stop sneaky jungle lane ganks and reveal supports.

MF's R "Bullet Time" Is the single most damaging ability out of any ADC in the game. With the Ability to Shred Squishies as well as Tanks, you can always rely on it to do work and provide a healthy teamfight for your team.

MF IS a Unique and Powerful champion, but is also lacking in many important abilities that help traditional ADC's Keep distance and provide Engagement.
Such as a blink. Without a blink getting out of a sticky situation can make you force a flash,
Which is another reason why Positioning is so crucial on MF. Unlike a champion like ezreal who can utilize his Arcane Shift to easily disengage and reposition himself mid fight.

IN Addition to her ultimate being her most valuable and damaging ability. It's also interruptible. Any Knock up, Stun, and virtually every type of crowd control in the game can cancel your ultimate.