Rocket artillery vs Infantry works well but level 4-5 artillery stacks also does significant damage against unarmored units.Artillery works well against heavy armour.
Destroyers vs Submarines yes.
Research will depend on your map, doctrine, enemies and your game plan.
If you have some idea how the game mechanics work it's a good start.The rest of your experience and skill you will develop and master by playing the game, reading the Call of War wiki,engage in forum discussions and taking notes on what experienced players are sharing and doing.
The best way to learn the game is to play the game.Don't quit if you have unsuccessful campaigns early on,we learn from our mistakes.If you can identify your mistakes it will be easier not to make them again.
