^
Step one: Do not focus the Rammus.
Step two: Do not give Rammus an easy path to get to you. His powerball may make him able to catch anyone in a flat out race, but a single minion gets in the way and it ends rather quickly.
Complaining about a Rammus who's making you feel more hurt than you're making him feel is like complaining about an ADC being able to crit 70+% of the time. It's what they do. It's the role they're designed to play. The counter play? Buying more health or more armor, but that isn't guaranteed to give you victory.
Wanna beat a Rammus? Buy a Last Whisper and as much lifesteal as you can, and hope you can sustain through the damage his shield is doing to you. Only that doesn't take into account that he's going to be hitting you for roundabout 200ish AD full build with his W active, or that his ult is melting your life bar away.
Some champs just happen to be potent duelists. The counterplay is not letting yourself be caught out alone which, if you're the ADC, you probably shouldn't be during the late game.