Campaign Mission 3-20 it's kicking my butt. How to beat it please?

Try to draw out the heavy tanks just a couple at the time,. Move your infantry to the front activate bunker and a couple of tanks will come then use all units to attack.

You have to be quick too, so smash through the first battles before you get to the heavy tanks then you may have more time to draw them out a couple at a time, Its important to focus their fire on your infantry that are in the bunker and save your tanks and other units which may have a weaker defense but deal a lot of damage,

Use riflemen and set bunkers to protect your missiles and howitzer.