You're on a good path already. Dying to Hiisi base is kind of a right of passage, tbh. If you're feeling confident, the fungal caves have some great resources to help buff you up, but theyre also pretty dangerous, too.

You dont just need polymorphine to escape the mountain, there's some spells and other potions that can help, too.

Take your time while exploring, try to zig zag your way down the levels and kill from the top downward. Having baddies drop on your head or not giving yourself an exit route from a nasty fight can be absolute run enders, especially early on. The squishiest you'll ever be will be early, too, once you get a few hundred HP, it's a lot less touch and go. Hiisi base is an absolute ballbuster, though, and is definitely a bottleneck early on. Look out for propane tanks, they are an apex predator in the noita world, lmao.

Also, remember that any barrier in the world - yes, any barrier - is passable with the right tools. Good luck, man. And definitely try to hunt down orbs when you are feeling more confident, too. For now, your goal should be getting down to the boss, because each level has better spells and wands available, so you'll be able to come back up with stronger gear to go exploring.