To get this achievement that's called "Come And Get Some, I Dare You!", you just need to fight ten online matches.
They can either be ranked or casual matches, it doesn't matter. It also doesn't matter if its against friends on your Xbox Live list or random strangers that challenge you. As long as you play through all ten matches and either win or lose (doesn't matter either), you'll get the achievement in no time. I'm unsure if you can change the settings to round one to make it easier, so if someone can confirm that or not, please let me know.
Have fun and good luck!