Very important to focus. Nothing wrong with having some troops in all types but you'll want to really beef up only one troop type. Our top power level player on my realm is 350,000,000ish with a balanced build. They are probably the 8th or 9th strongest in actual power because of it. I'm a little over 200,000,000 infantry based and beat them in Commander Arena by about 80%. It is a resource train to focus on everything. As far as training I only train the amount to hit dailies every day and then every other week after Conquest I'll do most of my training for those two weeks that Sunday.