Well, I beat the mission too with a little more than 400 points. It wasn't perfect, as I had one casuality. Took a screenshot but forgot where I put it, sorry. Will upload it when I find it or try the mission over again.
My lifesaver was the proximity grenades. Once I positioned my men along the sides of the radar choke point, I had them toss a proxy along the far wall where the Cyberdiscs and Sectoids would congregate and move back. The man inside the lower radar room would then move which would set off the proxy, kill the Sectoids and hurt the Cyberdiscs. Rinse and repeat until Cyberdiscs are gone.
How did you kill the cyberdisk to the right of the lower radar? I had a hard time getting anyone in a place where they could throw a granade or launch a missile that could hurt that cyberdisk. It's reaction fire was cutting everything down. It doesn't seem to move and it's in a spot where it can fire on you before you have a line of fire back.
Killing it was the key to winning for me. I had to sacrifice the tank/cannon to it's reaction fire and rush the tank/rocket into position on the same turn.
Then it was just a matter of bringing some reinforcements forward with a fresh supply of proxies. They blanketed the Access lift with proxies and the mission was over when the final Sectoid stragglers came to investigate (Commander and Leader I presume since some of my men were getting panicked every round).
Just panicked? I had two or three people pretty much constantly mind controlled. If I don't drop all the launchers on turn one half the people blow themselves up in a panick.
To kill the invisible Cyberdisc in the General Stores, I had my men upstairs just shoot down a bunch of large rockets. It fell after about 3-4 indirect hits. HE packs would have been better, but oh well, it got the job done. In my unaltered CE version of the game, large aliens which spawn partially in dirt are visible. I'd assume you have a different version of the game or are using XcomUtil or perhaps it is a glitch.
You can hit one of the squares it's in from the top floor if you kneel. Though, I had bad luck with even with kneeling-aimed-shots. Some stray missiles killed some of the people firing them by exploding on the top floor. In one try I used 10 missiles and still it didn't die.
I am using the DOS version and XCOMUTIL so that you are correct about that.