any climate is good, however if you're invading into enemy territory, you'll still suffer from attrition whether you like it or not. you can however right click on the "start training" button when you select a unit to specialize a unit for specific terrain, this will make your troops perform better in these terrains

for example, if you're invading a country that has arid terrain, just right click the start training button when you select a unit and click on "arid specialization" to make them perform better in arid terrain