Note: This little guide assumes you follow the exact Rune, Mastery, and Item build. Designed for SR 5v5 not ranked.
An alternative to the standard AD build that everyone uses. This build essentially makes MF a great AoE damage dealer, especially efficient in endgame teamfights. She also preserves the damage potential of her poke (Q), by taking advantage of many of the game's hidden or unsuspected mechanisms.
The build i am gonna describe deals alot of damage in relatively small ticks, making the enemy largely underestimate your damage, with the extra fun of sometimes making them stack Magic Resistance because they think it will reduce your damage.
Except MF is THE champion who now builds physical damage from Ability Power!
Preparing the fight
So we basically take maximum hybrid penetration, that's the important part. Yellow and blues aren't very important so take whatever you need if you find this build hard to play. Since i am extremely careful to not get hit by anything most of the time, i take Gp10 instead of the traditional full armor runes (won't have HP to synergize with them anyways), and scaling Mres because most long range attacks (those who get to this MF) are magical.
Once again pretty straightforward, maximum damage of both types, without the autoattack-specific ones like lifesteal and crit. I prefer the flat scaling AP than the percentage scaling AP because it's useful sooner. The one spare point is for your Flash summoner spell (It's in Defense on the pic, but in my actual page it was in Utility).
As a carry, you go bot with the support. I haven't tested the other roles.
Start with mana crystal and 2 pots, skill E (Make it Rain). Like any normal carry, focus on farming your cs. Alot will depend on what support you are with: if it's an agressive one, don't draw any aggro until your support initiates something with a slow or stun: that keeps your passive movespeed up longer, allowing a few extra shots on the enemies. Whenever useful, E them and start shooting, try hitting both with Q too to maximize the damage, but in any case focus their carry. When in doubt about enemies in a bush or something, Always scout with E first, no random facechecks (too squishy for that).
Note that level 1 E can help you farm some low HP minions at this point, but it can also make lasthitting minions harder if they come under tower after that (full hp melee minion is easy lasthit, just wait 2 tower hits, but when they're damaged beforehand it's harder).
As it gets to midgame, your goal will be to finish Tear of the Goddess for free spam and higher AP on the long term, Sheen for an extra proc damage on your Q, and Boots of Swiftness that you need for positioning (especially for Q and R) and not getting caught, because it's a very squishy build, more than the usual carries who stack Dorans.
The good part however, which is the strongest point of this build, is what i call the "Deathzone" combo: as soon as you reach level 9, you have the strongest E (not counting AP ratio) and your ulti. In teamfights, you want to chain them exactly like this: place the E over as many enemies as possible and immediately shoot R, covering the E area. This will usually destroy any squishies caught in there. The rest will simply take heavy damage and be slowed. I have witnessed full health AD and AP carries who died from just this combo, this is your goal. Carries who survive take a Q sheen proc in the face.
Now get Hextech Revolver and Vampiric Scepter into Hextech Gunblade to increase your overall dmg output and most importantly, to heal from everything. Midgame is about using Q-Sheen procs as poke, using E to slow enemies whenever useful, and farming: E will clear creepwaves almost on its own, because you level it first and have extra AP. Use W in trades to disable their healing and deal more damage, or when not fighting, to charge up your Tear. Also remember to chain-slow with Gunblade active into your E (or E into Gunblade if the kill isn't guaranteed, because the active has longer cooldown) to make them regret crossing your path.
According to leaguecraft's builder the total deathzone damage with Sheen, ToG and HTGB, level 9 is:
R 862 physical damage + E 393 magical, total = 1255. +Q sheen 190 physical (+some low magical).
Your HP at that time would be 1200, so if you could damage yourself, you'd barely survive it because of resistances (but anyone would finish you easily, which is why this combo tends to end teamfights). It's still not that impressive yet compared to the full build result (we'll see that in details later in the guide).
After that, decide which to get first: Rabadon's Deathcap to pump up your damage seriously, or Nashor's Tooth if teamfights tend to happen often and you need your slow or massive damage combo ready anytime.
After you get one of these, finish Lichbane for stronger Q proc (about +300 dmg compared to Sheen at this point) and movespeed. with MF's passive, swiftness boots and this, you have one of the best speeds in the game: 426 MS, not couting MF's passive. If they still get away, just ghost, E them, Q in their face and activate W for more free hits.
Note that your AP will increase your Q-Lichbane proc, but also your W's on-hit damage to greater levels than Wits End and Malady combined, because the AP ratio stacks 4 times! The AS from Nashor's help getting the stacks faster, and the huge CDR makes the AS steroid available more often, among other things.
At this point you have 1 main goal, if you reach it you may just have won the game for your team: landing a full deathzone combo on at least 3 enemies in a teamfight (don't go for the tanks preferably). Just farm your stronger items until you get a good opportunity. When you see one, run to a good spot and unleash hell. Very often that will involve having someone from your team trying to escape enemies, so they're gently grouped behind him, which will be their greatest mistake. For best results put yourself in a bush behind a wall, or something like that. Baron and Dragon attempts are equally great setups for a deathzone.
According to leaguecraft's builder the total deathzone damage with full build (not counting elixirs, baron buff, and auras like WotA):
R 2170 physical damage + E 807 magical, total = 2977 damage. +Q Lichbane 878 physical +45 magical.
That's AoE damage, that you spellvamp (or lifesteal, for Q), on slowed enemies, in a total of 3 seconds at most.
For the record, if you land the full deathzone plus one Q (aka your whole burst) on someone, that's 3900 damage and you can start adding the other 3 W stacks to autoattack for 180 magical + 122 physical, and a +621 physical every Lichbane proc.
The good and the bad
-The hilarious effect of watching your enemies stack Mres in vain,
because your AP increases your physical damage on R and Q.
-Extreme farming speed.
-Solid damage both magical and physical, making only balanced tanks hard to kill.
-Great movement speed for chasing, escaping, and positioning.
-No more sustain or mana problems as soon as you have ToG and the base parts of HTGB.
-Teamfight-winning 2.5 seconds combo if you're not interrupted, and great spellvamp from it, making it easy to not die during the 2 seconds channel if not enough enemies focus you.
-Extremely squishy, you want allies with CC and a support who wards.
-Assassins like Nocturne and Talon.
-All-offtanks teams, not so rare in this meta. Because most may survive the deathzone, and they'll want revenge!
-The most damaging part of the combo is channeling, even though it's only 2 seconds. You need to make sure you're not about to be hard-CC'ed.
ConclusionHere is a little example picture of the results against some appropriate enemies (namely: not tanky enough, and no healing support during laning)
Comments and constructive criticism welcome :-). Please know what you're talking about (If like many persons, you think spellvamp only works on magical damage for example, learn how LoL functions before saying anything, thank you for your understanding).