Problem is NOT solved by the bloodrazor actually. Bloodrazor sucks. If they have lots of armor to mitigate physical damage, chances are they have a good deal of magic resist to mitigate magical too. And you have ZERO magical penetration as an ADC.
So with 200 MR, bloodrazor is adding a whopping 1.3% of their max health per hit.(1/3rd of 4%) Wow.
Plus this damage is magical, not physical so it doesn't even work with your crit. Why spend all that gold in an Inf Edge, have high crit chance, 260% crit damage, lifesteal, and then get magical onhit damage that doesnt scale with crit nor work with lifesteal?!
Its just dumb as hell when you think about it. ADCs already outscale tanks significantly.
With 300 AD, 2.0 AS, 50% crit, your DPS is: 300x2.0 = 600 plus 50% chance of 160% extra dmg = 80% of 600 = 500.
Total of 1100 DPS. 400 AD with 100% crit @2.0AS is 2080 DPS!
Even with hundreds of armor, you cut it in half with Last Whisper and your APen, then deal sooo much damage that they still melt pretty quickly.
Deathfire grasp isn't really an anti tank item so much as a per-champion basis item. It is especially good for champs that can make use of its properties, being CDR, MP5, and need the additional nuke. AP Sion is a great example of a DFG user.
Void staff is really what you want. The 40% penetration is huge and significantly increases your damage.
But honestly these help but aren't counters. After all, do you really expect there to be items that allow you to totally negate tankiness?? I mean, what would the point of being a tank be, if the enemy can just buy 1 item and negate it all?
Instead of worrying about the tanks, SHOOT THE NON TANKS INSTEAD!