30.000 first level fighters... Let's say 20.000 of them use missile weapons. Sure, many will be firing over a distance giving them a severe penalty, and they probably don't have more than 30 or so in OB, but: 20.000 rolls gives 1.000 open-ended. Oh, but the distance penalty, low OB and defenders DB makes sure that's still not enough. But it's also 50 double-openended rolls. I reckon THOSE might do some decent damage.
And in melee: Even if you whack one of the 1st lvl dudes every round, expect at least one or two to be able to get in an attack on you every round. They may need an open-ended roll to get a decent crit (but not double-openended), and even then, it's probably only around 20-25% chance that the crit will take you out. But that still gives them 1-2 % chance of getting a killing blow every round. That means the 300 10th level guys will lose 3-6 people every round - if they are all in melee. Sure, they will be killing off 300 1st lvl guys every round in the start, but as their number diminish, their kills will too, and the army of punies will eventually emerge victorious.
You'll definitely need extremely advantageous terrain to stand a chance against such an army, and even then, it'll be hard. With a fortress, sure. With magic, I guess maybe, but still you'd need decent terrain advantages.