ADC's are the ones designed to shred the tank. And yet, 5 seconds is a long time compared to the split second that an ADC would last when focused.
LW is really strong at countering Bonus armor, Yet you really need to combined Giant slayer with BotRK to health shred. So Under the current meta, I would advise more HP over high Armor. Also, If Ashe was behind her team, I doubt she's the only one attacking you. 500 Armor is a bit overkill, but 3.5k HP can be bolstered a little.
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Because armor items are useless come late game, you need to build damage, its all about damage now.
On Rammus, Armor is damage. He gets 25 AD for every 100 Armor. Dueling the ADC shouldn't have been a problem if he could get on to them. With that in mind, Rammus most likely wouldn't be trying to attack the ADC while her allies are defending it. You should be looking for a flank opportunity, And Ideally, Ashe would have been dead long before her allies figure out what went wrong.