En Sabah Nur
The First
Ohhh. I've seen that melee upgrade, just never bothered to get itlava walking is an upgrade in biome 5. its position is fixed; you just have to figure out the location.
getting through yellow walls is much easier: you just need to buy or find a proficiency upgrade for your melee weapon (the upgrade looks like a lug nut). the upgrade doesn't tell you that's what its for, but once you have it you can break yellow walls with your melee weapon.
that said, if you grab a parasite or pick up a malfunction that downgrades your melee proficiency by 50%, you'll lose the ability to break the walls again.