You will get a chance to fight the Tsar Tank in the 3rd stage. He will be the final wave that comes out. Make sure to build all your troops with the highest ranking possible. He stops and drops enemy troops off so you will get a few chances to control any gunner to weaken him alot. Once he gets into your base, he will destoy your placements, so just get ready to repair or replace them quickly.
I destroyed him when he was approaching my toybox, so its not too difficult on the casual setting.