You need to win one round in a ranked game where you have 10% at the same time that your opponent has as close, if not exactly, 100% health. Seems to be big debate on the internet as to the exact percentage, I can confirm that if your opponent is at 50% when you reach 10%, and you then succeed in killing him, you will NOT get this achievement.
Best way to boost it is to get into a ranked game, and send a PM to the person you're playing against. With much luck, that person will want to boost the achievement with you. At the moment this seems to be the only viable method of boosting, as matchmaking with someone you know in ranked games is next to impossible. Both of you can get the achievement in a single ranked game, you only need to win the round this way, not the match.