The first thing to do is kidnap a fairy for information on how to get to Thorn. After all, all he knows right now is that his target is somewhere around here sometimes.
He strengthens his senses of sight and hearing, and takes a quick look from behind the waterfall for whether there's a convenient place to lie in wait. One that wouldn't lead straight to the portal. Zinc makes sure he'll fully notice everything within his field of vision, and chromium protects him from being seen.