Ummm.... as a Rammus player I gotta say, it's not that hard to beat Rammus. He hardly does any damage if you don't hit him, and if you have lifesteal that shouldn't matter much. Rammus is only dangerous if his team is with him. Kill them first and he'll try to run away, but you'll probably kill him when he does. (just don't auto-attack him when you're low unless you want to die too.)
Oh, and if you get really really REALLY close, he can taunt you. (Seriously, the shortness of that taunt range drives me crazy. You have to be face-to-face to get taunted.) If that happens, you'll auto-attack him, with broken armor, even though attacking him hurts you. So don't get that close. Kill his ADC instead. If your ADC is dying first, make sure somebody is throwing slows on Rammus while you're killing his ADC and other damage dealers. Then you can kill him in peace, thorns or not.
When I'm playing Rammus, there's nothing worse for me than people just -walking around me- like I don't matter and killing my damage dealers, and I can't do anything about it until my Q (that's the speedball) comes off of cooldown.