Unfortunately, the quest "I'm Probably Dead" late into Plankerton proves to be the toughest quest I've ever been. It appears no amount of defenses I build will work because the building has many entrances where Husks can simply creep through to attack the servers. A quick look at some walkthroughs online reveals that players should exploit how only at least one server needs to be saved to clear the mission and quest and I would concede with that here. But I would like your thoughts on a new plan I've come up with to beat it solo that I have yet to try:
Focus on defending servers A and B because they are on higher ground.
Go to the site of the lab explosion. Then visit servers A and B. Do not go down to the other two servers, let the Husks destroy them.
Making sure you did not see servers C and D, the 5 minute countdown for the attack should not start. Demolish the surrounding area around servers A and B so that there is virtually nothing around them.Build the double-wall testudo/doughnut of death around both servers. Call your three Defenders on the top of your fortification with Defender Posts installed on the rooftops.
When you are finished, go downstairs and look at the remaining two servers so that the 5 minute countdown begins. Then see if you can protect the two servers back upstairs. (Protecting one server requires fewer resources, but increases your chance of mission failure here.)