Copper and tin are easy to get. Just need the services of Troll Inc.. Just need enough to craft a bronze pickaxe. The major problem is getting iron after that. Iron outside of crypts is REALLY hard to come by.

I'm currently having that problem, so to speak, as I'm on a "no portals, no bosses" run myself, and I just reached the swamps. I know some mobs do drop iron, but the chances are almost nil for that. And hammering the entire swamp with a Stagbreaker just to maybe get a rare chance at loot underground doesn't sound like a lot of fun to me...

So, not sure how to tackle that one yet.