I'll go with the flow
And say what has to be said, if yet just another time:
This is too hard, tideous, and the mob invincibility just makes it boring to play, it doesn't even add a proper challenge. It's just... hard. Too hard.
I couldn't even bear myself to try and kill the A-Bot, after dying over and over again on my way there.
See, I have no problem with challenge, but when it comes to you essentially being weaker than the average enemy and you're up against tons of them, it's not a fun game to play.
2 for the concept, but I'd really like you to play though your next game before submitting it, and thinking: "How hard is this, actually?"