In terms of itemization, buy some armor and hp early on. While this might seem obvious, there's a little more finesse about it that people often overlook. You don't have to go balls deep into defense itemization (ex rush
Randuins or
Sunfire first buy), you just need enough defense to reasonably survive the incoming damage in order to retaliate (in which you will need to be able to deal a reasonable amount of damage back at them).
I often buy an armor component, HP component, and damage component (not a full item, just parts for the stats) in a varying order to slightly boost my ability to absorb damage as well as return damage. From there, I look at my lane situation to see what to heavily invest in. If the enemy laner is pushing out a lot of damage, I'll buy more defenses. If they are pushing out a manageable amount of damage, I'll consider either defense or damage item. If they are pushing out less damage then expected, I'll go damage item.
In terms of gameplay, it depends if their kit is targeted or skill shot oriented. If targeted spells (like
Pantheon or
Fiora, I know it will never miss and this places more emphasis on items. This causes me to play more cautious and passive, as we are playing an itemization race at this point. I'm guaranteed to take the damage so I'd better have the items to be able to take it (can't fall behind due to deaths or recalls).
If skills shot spells (like
Zed,
Riven or
Renekton in terms of spells dependent on their position), I know there is less emphasis on itemization and more on skill. This gives a little more freedom on items as it's less of how much a damage a spell will do verses if the spell will actually hit. At this point it entirely comes down to personal preference, as the player would know best of how likely a spell will hit (their skill level) and can build items accordingly.
So, the original question. How do you defend against a
Brutalizer rush? Well, the answer would greatly depend on the situations outline above. It is important to note that you can entirely influence the rate of acquisition the opponent will get the Brutalizer (advantages trades, zoning from farm, ect). You can also influence the rate of your own item acquisition (trading again, strong CS mechanics) in response to Brutalizer, which would then influence the effectiveness of it. Additionally, you can consider outside influences (your team and their team jungle/other lane ganks, Dragon buffs, and hell even a full on lane swap with other lanes).
tl;dr : Depends, I got real deep bro.