Hidden Quest

If you've cleared 3-5, you can definitely clear 4-5 too. Try looking up guides and take it step by step. If you don't give up, a path will surely open up!

Mech levels are shared, so it's easier if you consistently level them up as much as you can afford and decide on a style to guide your upgrades. For the basic and scorpion-type mechs, focus your upgrades on splitting + quantity + refraction. For tanks, focus on AoE (area of effect) upgrades. It's good to upgrade about 3 of the main weapons you use to around level 15-20. Specifically for the saw blade weapon, I recommend upgrade for bouncing on the basic and scorpion types, and for tanks, upgrade for slowing down enemies.

I was also aiming for 3-5... ㅠㅠ

I'll try the recommended method above too.