Wait, how do I see where the reinforcement holes will be?
The first turn's enemies come pre-spawned, so you already know what they are. Further enemies you can either block, or wait until they spawn and then kill. I mean, you have a turn for reacting to them
after they spawn. You should be able to reach them with
someone unless you're bunching all your units in a corner of the map or something. Artillery units alone can pretty much cover the whole map by moving laterally then shooting.
You don't need to watch a whole 1 hour walkthrough, but if you're losing in two stages, then the first two stages of the video will already play drastically different. A good exercise is, watch the turn layout, pause the video, think about what you would do (and whether or not it would protect all the buildings), then unpause and see what he actually does. You should plan the entire turn in your head before you make the first attack.
Eh, you're savvier than I am if you can pull that off. I've always found subpar deployment to be the difference between a perfect run and being forced to soak grid damage.
Soak one or two building damage (with a really bad deployment), sure. Lose the entire stage? Not really.
Deployment is pretty simple, really. Put your melee units on the front, brutes in the middle, artillery in the back (ideally in the middle of an unobstructed row). Try not to bunch your units together if there's spiders and such. There's really not much more that you need to be mindful of.