No OS tuning, stuck at Defend the Dam. Help.

What it says in the title. I’ve beaten and S ranked this game. Now I’m doing a challenge run where I don’t do the tutorials and I don’t get any OS tuning upgrades. I’ve had to cheese one or two missions, but so far it’s been challenging and fun! But now I’m stuck at the Defend the Dam mission. It’s brutal, and I can’t finish taking out King and Char with any repair kits lefts over. My time to kill is just too slow, and my defenses can’t make up for it.

Any advice? Builds that don’t require a weapons bay or kicks? Ideas? Strategies? Literally anything, I’m completely stuck.