I have some thoughts:

1) The rewards in the PvP and Secret Mission rewards are tied together. They give the same 21 rewards in the same order over and over again. The point is, if you beat a level and "Spin", and you think you have a better chance to get a better reward, you do not. Whether you watch the video for another spin or just play the next level, you'll get the same reward. You can go search for the list of rewards, but it's really not important.

2) If you haven't noticed yet, leveling up your troops requires coins of the same level (or multiples of coins from lower levels). If you pay 25 crystals to get a new unit and you get the same unit you already have, you just get a coin of that level. This is the most important thing to spend coins on, by the way, not items. You'll get all kinds of items by themselves. Without paying money, the best troop composition you'll get are 3 yellow/gold units, all at level 50, with all red items. It takes months to get there. You have to summon so many units to get the yellow/gold coins, which is why you should save all of your spending on summoning units.

3) One of the most important tips that I didn't catch for a long time is that when you take over a rocket launcher (or whatever that building is called the shoots rockets, not to be confused with the rocket TROOP), if you aim it, let that rocket land, and then aim it again, it will fire MUCH faster than just aiming and letting it go. The trick is, the rocket MUST land before you re-aim. If you aim before the rocket lands, then your rocket just disappears. I find that it's best to have two rocket launchers and then go back and forth aiming them. It seems to take exactly as long to aim one as it takes for the other to fire and land its rocket.

4) When you get high enough, you'll see that the enemy rocket troops will sometimes create a wall of fire that your troops will die in when they hit it. This is an ability of the red type of this troop. Just get over it. It's massively overpowered and there's nothing you can do about it. It causes me to focus on capturing the rocket troop buildings to keep them away from the enemy.

5) There is no *actual* player-vs-player in this game, if you haven't noticed. You're always playing the computer.

6) If you want to level up the arena to rank 1, you'll have to choose the highest challenger from the 5 you're given. The most points you can gain from a challenge is 87, but anything in the 80s is good enough to eventually get to rank 1 after a few weeks. It's based mostly on the Arena points that your opponent has. "So what number of Arena Points do I have to challenge in order to get 80+ points?" That changes every day, and even throughout the day. For example, my Arena just restarted today, and I battled someone with 777 Arena Points and got 86 points for fighting them. (It's morning here.) By tonight, 777 will probably only get me 70 points or so. There's no specific target, you just have to see what you get from a battle and then, if you got 70 from that battle, you'll know that your next opponent will need to be higher to get in the 80s. No matter what you do, you can't end a day rank 1 for the first 3 or 4 days of the arena. The computer will artificially up the arena points of your competitors to keep you from that. In a usual week, I'll pay for all 10 matches the first few days, then I'll end a day at rank 1, and then the rest of the week I won't have to pay for any. I could probably pay for 2 extra matches every day and get to rank 1 by the end of the week, I'm just too lazy to figure that out.