This riff, although not polymetric in nature, is a very common pattern in metal. Lamb of God and Meshuggah should very well come to mind when playing some of these 32nd beat gallops.
We learn early on that alternate picking usually entails a down-up motion, regardless of string/note transitions. Well, there are certain exceptions, especially when it comes to achieving greater speed. Throughout this 4-bar riff, you'll see a common rhythmic sequence common to a "gallop", except with an extra 16th beat added. Most people attempt this riff using the following picking pattern:
Let's try this, instead:
This WILL take practice - and the upstroke on beat 3 will probably be the cause of confusion. But with practice to a metronome, it'll quickly become the most economical way of achieving the speed required.