Mechanical zombie: priority for upgrading parts

Mechanical zombie.
What to download?

Level 1 head = 0.5% combo (double shot)

Level 20 head = 1% combo

Level 50 head = 1.5% combo

Level 100 head = 2% combo

This applies to all parts, every time you improve

There are up to 20, 50, 100 levels, this enhances the additional effects of the parts (lethal damage, critical damage, etc.).

And up to level 100, you only need one part to improve that part.

Torso - echo (double cooldown)(0.5, 1, 1.5, 2%)
Hands - lethal damage (30, 60, 90, 120%)
Legs - Crit damage. (6, 12, 18, 24%???)

Skills.
The best skills are red. Which give +10% attack bonus.

You can open the skills and read which one suits you best. There are against bosses, there is for elemental damage.
They all raise the common BM.
You can distribute it evenly, you can pump only those that you consider beneficial for yourself.