It's actually quite manageable

After trying out the zombie-related Mech Assemble and Zombie Waves, I found zombies to be much easier than mechs. It's really tough to beat each Mech stage, but with zombies, I'm on my 70th run and haven't hit any roadblocks. I'm using the Wind Rifle given through missions and upgrading it, and it's been no problem at all. Diamonds are just piling up, and I'll soon have 100,000 lol. I've completed all the reward missions and am considering whether to continue. There are various unit facilities that significantly boost combat power, and one of the robots can freeze all visible enemies on screen for about 5 seconds, so there's no risk of dying. The fun of slaughtering is quite addictive.