The VIP Shuffle

 Missions are the best way to gain materials, resources, loyalty, and commander experience, all of which are critical to upgrading your commander's skill set.  There are four types of missions available: Base Missions, Daily Missions, Alliance Missions, and VIP Missions.  Base missions are awarded for achieving milestones in the game (like new structure levels, troop counts, etc), and are not the type of mission I'm going to focus on in this post.  The others are made available every six hours.

An active VIP will give you additional benefits when it comes to your missions... most importantly are bonus missions and autocomplete functionality, so that you don't have to wait for timers to complete before starting your next mission.

A player will earn VIP days almost automatically via the challenges, as long as they are active.  These typically come in 30 minute, 60 minute, and 1 day varieties, with options to purchase 7 day and 30 day varieties.  When you activate a VIP day, immediately, a number of missions in each category become available.  When they expire, you lose whatever missions you haven't completed yet.

This leads to a tactic I'd like to call "The VIP Shuffle".  When you complete all missions, allow your current VIP session to expire.  If you add additional days to your current VIP session, you don't receive any additional missions... but once they expire, you'll typically get somewhere around 10 new missions when you renew VIP status.  This actually makes the shorter VIP periods more valuable, since your goal should be to complete as many missions in as short a period of time as possible.

Consider a 24 hour period, where on the one hand, you use the "1 Day VIP", and on the other hand, you activate ~47 "30 Minute VIP".  At my current VIP level (9), that means in the first case, I'd receive 23 missions 4 times in that day, for a total of 92 missions.  In the second case, I'd receive those same 23 missions 4 times a day, plus an additional 13 missions 47 times, for a total of 703 missions.  Effectively, that means your commander levels up at 7 times the speed.

This can further be refined, by realizing you can start a "1 VIP hour" 30 minutes before one of your 4 daily mission bumps, to first generate your 13 basic missions (which you complete in 30 minutes), and then get your 23 regular missions on schedule, and complete them in the next 30 minutes.  If you do receive an advanced, elite, or legendary mission during this time frame, you can always extend to a full day so you can reap these rare rewards.

In my opinion, this has been the most effective tactic to grow my power rapidly since I started the game.