As far as I've made out (since my first post!), it just goes up faster by playing Battle & Team Battle (the former is faster than the latter!). I think War makes it go up the slowest (unless you are capturing a lot of positions).
Ever notice how if you join a custom battle ranked match, most times it's dudes with ranks of 28-36 or so? I think it's the best way to boost your skill ranking, unless of course you're constantly getting pwned! z;)-~