First and foremost it is your HQ. Leveling up your HQ to 30 should be your number one priority. You should not delay leveling your HQ for more than a few days for VS points on days 2 and 5. Increasing HQ level increases not only your power but also the contents of your EXP chests and your resource chests for food/iron/coin. Get your HQ to 30 as quickly as possible and then work on getting everything else caught up. At HQ 27 you get access to higher level secret tasks which grant greater rewards and MEGA. At HQ 30 you get access to even greater rewards and can participate in higher level activities.
Second is getting your troops to T10. Get to the bottom of your Special Forces tree as fast as you can but don’t forget to level up your Tech Center and Barracks to 30. Leveling up your Gold Mines to 30 helps a little with this. Also, Engineering skills “Rapid Production” and “Instant Gathering” are helpful ones to max out.
Third is your hero levels but do not delay leveling your HQ for that. Hero levels will catch up quicker when your HQ is 30 as your EXP chests will automatically grant you more EXP. Focus on your squad #1. Get them to 150 one at a time, don’t spread it out.
With the hero stars focus on getting one hero to 3 stars before moving to the next one. For instance, get Kimberly to 3 stars then get Murphy to 3 stars and so forth. Then get squad 1 to 4 stars one at a time and then 5 stars. Once squad 1 is all 5 stars move on to squads 2 and 3. Do not worry about the wall of honor until all 3 squads have 5 star heroes.
Hero skills are the hardest thing to level and here your can start spreading out the skill medals a bit. Level up skill #2 first on your heroes (numbered top left to right then bottom left to right so for Kimberly #2 is “Barrage Strike”). Start with your attack heroes and then your defensives. For tank squad that would be Kimberly > Stetmann > Marshall > Williams/Murphy. If tank is your squad 1 then Marshall skill #3 Rapid start is your next priority. Beyond that, read the skill and use your best judgement or ask in alliance chat. Your squads power will rise rapidly as you max out the skills.
Tech trees will also increase your squad power as well as speed up your ability to level up your HQ. While leveling up your HQ it helps to max out the Development tree and 90% of your Economy tree (you can ignore the last 3 in the economy tree.
It is important to level up your Tank/Aircraft/Missile centers to 30 even though they take a lot of time and resources.