You struggling to beat Poland's final mission with infantry only? I had the same problem and this is what I have found.
Look, you're going to need to invest in snipers and officers. In the tech tree your going to want to go to the ground units and research the Panzer 1b. It's a tank that only the officer can use. You may be thinking, "But it's infantry only!". Well officers count as infantry no matter the vehicle. So you will absolutely need to get these, or else you will have a much harder time beating the level. Your also going to want to research snipers. Because they are just a riflemen that can actually hit things. For your the officers you choose it's best to buy good officer with good perks (mind you, you can upgrade the officers perks with research points. But you can only upgrade the perks the officer starts off with i.e if the officer gives +10 rpm and attack you can only upgrade those things. You cannot upgrade damage reduction.).
Placement is key, Because otherwise your going to be destroyed by paratroopers. Your going to want a healthy 60/50 or 50/50 split of snipers and riflemen. Your going to want to place them sprinkled around and spread out a little bit. Otherwise polish paratroopers and mg nests will kill you right off bat. Your going to want to place them mid to back of the map or else again mgnests and paratroopers. you should place your tanks as you have place your infantry. But to keep your tanks alive I found it easiest to micromanage their healthbars. If their armour and health get low take control of them and take them to a generally safe area and repair them. The battle should be good once your over the mg nests and the paratroopers.
One of the VIPs is a Jeep itself. So if you notice one of your guys got a jeep. Get him out and destroy the Jeep. For some reason the Jeep itself is a VIP. Also you should use molotovs on building's to get the best damage.
Your good to go! You now have to blitzkrieg into France! The next couple acts are extremely easy (if you invest in the tiger and the panther.) I do hope that this guide found you well, and you are now not stuck on the arguably worst level in the game.