Minecraft Skyblock building down in nether

simple solution: piston pushing the cobblestone down.

less simple solution requires the witch to throw fire resistance on herself, you being affected because of standing near her, than running through the portal and lava-scaffolding.

You are able to get pistons before getting a villager. You are able to get Iron from killing zombies, but the redstone is a bit trickier. The villager method is definitely the best way to do it, but there is also the possibility of finding a place where a witch is supposed to spawn (witch hut) and kill witches for it. They spawn naturally without huts, but it is quite rare, so that is another option. It shouldn't be too hard to get 1 redstone this way.

As for powering the piston, a button works, which is very cheap. You push the blocks down little by little, then you can make a stone platform to catch everything you need. Good luck to anyone playing now, Skyblock is an underrated game mode. Remember, Skyblock for 1.16 has recently come out, so check it out!

You can try suicide lava scaffolding and repeat, instead of straight jumping off. Alternatively, for the piston method, remember that iron bars are also a rare drop of zombies. If you have a decent mob spawner set up, you can weaken zombies by a deep fall, while avoiding to kill them. Then you hit them once with your sword to enable the possibility of getting a rare drop. After a while you should be getting an iron bar. You luckily only need a single one for a piston. Witches can provide the redstone for the piston.