Boss character
Each level has a boss character, and beating them requires you to watch them for their patterns. At first, focus on avoiding their attacks. Don't get eaten by the T-Rex in the jungle temple area, for example. Watch for its vulnerable point and then shoot at it. Repeat this until you kill it. There is a pattern with all of the bosses in this game.
Collecting diamonds
Diamonds are rather tough to come by, while gold coins are easy to come by. Collect gold coins by running in levels, and like gold coins, collect diamonds the same way. You have to get somewhat far in the level in order to get there, though. Also, when you're in battle, shoot at the barrels and crates in order to earn more bonus coins.
Earn gold coins
Completing achievements is yet another way to earn gold coins. Tap on the icon that looks like a trophy with three stars above it in order to get to the achievements area. Tap on any completed achievements in order to collect a reward, and read any of the incomplete ones to find out what you have to do in order to complete them.
In-Game obstacles
One thing that makes it fairly easy is that there is very little variation from one playthrough of a level to the next playthrough of the same level. That means that you can start to predict what obstacles will pop up in what area, and how to deal with the area where you died before. Pay attention to everything that you run past so that you can see what to do next time.
Boss character...