To raise your highest skill in Halo 3, you have to play ranked matches and of course win them. It's not based on a oh I've won 3 matches in a row where's my rank up? - it's based on Microsoft's TrueSkill system, which pretty much assigns you a rank based on how well you play. For example: You are skill 15 and go into a match and get 30 of your team's kills, and you guys win. Chances are, you will rank up - but this will only be true at the lower levels. It took me around 13-15 matches to rank up from 49-50, all wins with no losses in between. I recommend playing Team Slayer with a good group of friends, and you should rank up in no time as long as you win. Also, if you lose a match to people ranked MUCH lower, you will most likely rank down, if not be on the verge of ranking down. Same goes the other way around, if you win against much higher ranked people, you have a good chance of ranking up.