There is a funny trick.
If you spam 1 troop transfers and then attack every adjected enemy territory with one troop, there is a good chance that you will get some of the enemy's territories with that 1 troop attacks. This works, because the enemy will make attacks from the territory. He will very likely leave 0 troops on the territory after the attacks are done. If you attack the territory after he does the attacks, even 1 troop is enough to get the territory. More about move order: https://www.warzone.com/wiki/Move_Order
Bonus trick: if you have free troops, double amount of troops next to big enemy stacks. No matter how big the stack is, this strategy will prevent the AI from taking all but one adjected territory.