mountains are pretty simple.

Use a frost potion until you can make an anti frost cape or chest piece.
find like 2, maybe 3 veins of silver using the bonemass item, and dig it up, you know the drill. Silver is found above average heights and where obsidian is plentiful.
along the way, pick up 2-3 nodes of obsidian rock.
if you can find a cave, loot it for crystal (optional) -- it makes 'glass' window panes for decoration. Maybe do not go very far into these right now, just grab crystal for now.
visit all the shacks and weird stair-down-ruin things until you find a chest with onion seeds in it.
The boss summon item is exceptionally heavy and cannot be portaled. They are plentiful, usually, esp near the boss, so its rarely a problem.
everything else will take care of itself.
You can usually finish the biome very quickly if you can find silver.