Hey there; my name's SoapyWTF (I play in NA) and I carried myself out of ELO hell with this build; and you can too! This build will focus on a hybrid offtank core; and will situational items. (This is also my second build, don't hate :D)

Pros; Can jump in and out of fights.
Unstoppable if fed.
AMAZING kit.
Cons; Can be killed easily if focused.
Somewhat weak early game pre-6.
People will rage the **** out of you.

I take 3 Attack Damage marks for an early advantage.

The 6 Attack Speed marks will help late game, when you're jumping on the AD / AP Carry.

Alternates would be that you could get 3 Armor Penetration marks and 6 Magic Penetration marks, or all AD marks.

The 9 armor seals will help for the top bruisers; you could get mana regeneration per level seals if you're running OOM.
The magic resist glyphs will help early game if you're against a magic dealer; you could always get magic resist / level for late game.

The quintessences are pretty explanatory; an early advantage. You could always get flat health quintessences to be more tanky early game.

Masteries are self explainatory; you need survivalibility and a little bit of offense to jump in, kill the carry, and jump back out.

Ignite and Flash are your Safe Kill combos.
If you want to be super aggressive; get ignite and exhaust.
Don't get revive or surge... it's not necessary. You don't need clarity, if they have tons of cc get cleanse, if you're new get heal and flash.

Getting a early phage will help with trades; Wit's End will help for ganking and toggling your phage proc. The Trinity Force is the best offtank item ever. Then grab a warden's mail. That's just the core;
now here's the real items.
Randuin's Omen will help when you're jumping on the AD carry; if they fight back, there's a chance it'll toggle the passive; slowing the attack speed.
Guardian Angel: You have two lives to live? GG.
Frozen Heart: Dude, this will help ALOT when you're jumping on the AD carry; 20% reduced attack speed? :D
Hextech Gunblade: This will help alot with survivalbility; Spell Vamp + Lifesteal + Damage will help ALOT when you 1v1 people.