How I make troops do more damage

Use the right troops on the right targets. Ignore the "average" strength at the top on the unit description. Every unit is good at a few things and bad at everything else. So if your opponent is sending tanks at you, defend with AT guns, while bombarding him with artillery and attack bombers.

Use terrain to your advantage. Infantry and AT guns are stronger in forests and cities. Don't use them to defend hills or plains.

Stack your units. In general, don't use single units for anything. Make stacks of 10, both on offense and defense. Bigger stacks win faster while taking less damage. The worst thing you can do is send single units to attack far away targets. Don't do that. Move your units to a single point in the map like a city close too the front line, choose the best stack composition for where you're going, and leave the rest behind.

so what if the have every kind of troops and tanks and all that attacking me

Then use units with better defensive stats, because retreating is impossible, you can draw in their armies and get them into a battle they cannot win, if its mainly Tanks attacking you, use TD's, preferably in forests, if its mainly Infantry, its better to attack those stacks.

thank u both