I wouldnt make mercury Treads my default, with good position + swiftness they should only be taken on mad CC teams. I must say i like the inclusion of new black cleaver ive used it well in most games. I dont like frozen mallet, id rather grab banshee's for survivability and the slow can be obtained from red buff which you should have if your carrying.
You really want to throw in an Infinity in there aswell in my opinion. Runes you also want mana regeneration on blue, she needs it to cause some serious harrassment.
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Honestly i only really get boots 1 with MF only because her passive, but ill get zerkers/mercs/switfness what ever the situations calls for. as for her build I build similarly;
-my finished build, how u do it while u play is up to you.
-boots /merc/swift/zerker w/e u need depending on the game
-thirster/bloodrazor ( i only get bloodrazor if the enemy team is beefy)
-something defensive (banshee's/guardian/hell even a warmog's lol)