I've been playing a lot of ranked recently and doing pretty well, but the higher I climb, the more mechanically talented and intuitive players get (well, obviously) One of my ongoing troubles is the universal confusion and panic that is brought on in the late game when the ADC has damage, and now, penetration at which point the tank is chunked by what looks like AP burst. Are there any tactics or builds I should try when this occurs?
The best tactic is to simply do the job tanks are supposed to do, which is use your CC to either keep the enemy backline busy while your team handles the enemy front line, or peel the enemy frontline away while absorbing damage from the enemy backline. Your job isn't to survive, it's to last as long as possible and adjust the team fight environment into one that favors your team.
thornmail + health, + sunfire + 1 damaging item is enough to deal with any full pen champion.
if they are heavy AD then 
+ one item with MR and at least one offensive item. 
can work well if you want some AD or 
or 
if you're AP.
if they are heavy AP with less AD then 
or 
works very well in place of
Well since penetration goes through armor/MR you would build more health so you can take more hits, focus Adc/mid depending on who is penetrating you :P
I generally say there's no counter build to slowed attack speed and build randuins/frozen if I'm up against the sort of carry who tends to attack the tank.
