Tip 1: You'll come across a number of easy enemies on your run, but you'll come across many that are hard to kill and will deal a lot of damage, as well, such as the armored goblins. First, know who they are, then scoot to the side of the track and when they come face to face with you, move off to the side to avoid their attacks. Furiously tap the screen to hit them as many times as you can as they make their way over to you, then when they're close enough to do damage, move to the other side of the track.
Tip 2: Run far enough in the stage, and you'll hit a checkpoint. You can either pay gems or watch a video to enable it, and when you do, you'll start back over from said checkpoint the next time that you begin playing again. The checkpoints will be in different places depending on which twists and turns you decide to take throughout the level. When you start back over from a checkpoint, pay gems or watch the video again to use the checkpoint the next time after that.
Tip 3: As you open chests, you'll find a lot of equipment that's a higher level than you are, so save it until you gain enough player levels that you're able to equip it. Much of it will have some skill with a star meter, e.g. 0/150. That means that you have to level up the equipment by killing monsters. When you kill enough, then the equipment's side ability will be unlocked.
Tip 4: You can spend your skill points however you want to, but as you level up you'll unlock more different types of skills. Hit the reset button and you can spend a number of coins (not gems, thankfully) that allow you to reset all of your skill points that you've spent. Then you can re-spend them however you want to, so that you can get skills that will be more relevant to passing the tougher rounds.
Tip 5: Hit the achievements if you want to collect free gems. You'll have to double-tap each completed achievement in order to collect the gems from it. For the achievements that you have yet to complete, you can read up on what the requirements are in order to figure out what you have to do in order to get more free gems.