Don't stay on one side of the level
Enemies swarm in from the top of the screen but cover the entire area kind of like Galaga. It's easy from a strategy point to stick one side of the area and just fire straight away at foes, but it's good to move around and get extra kills.
In short; kill as many enemies as possible, and although this seems like a Captain Obvious statement, it might be easy to stay on one side and get easier kills to beat the level. But, by taking out more foes, you'll be able to earn many more coins.
Coins are important in the game because they let you level up your character. The stronger the better, right? I suppose if you want to challenge yourself a little more, you can stay at a base level. But, if you really want to boost your archer, and make them deal double the damage, then make sure to destroy as many monsters as possible.
Remember, you only have one life
Yes, you may feel like an unstoppable killing machine in Vikings II, but just remember that you have one life that's given to you on most levels. Get hit once, and it's game over. Thankfully, you can very quickly get right back into the action again on the level you're on.
So no, you don't need to start completely over again (many games tend to that) from the first level. Each level is like your checkpoint. You will need to start over on that level if you die though of course. But, this is OK, since you can get through them pretty quickly.
But, just because levels are short, doesn't mean it's not annoying to die and start over again. Under any circumstances in video games is that not annoying (ugh). So, just let the arrows flow and don't run into any baddies and you'll be golden. Simple, right?
Grab those power-ups
Another reason to move around the whole level, power-ups will drop quite a bit, give you an advantage on the battlefield. Most of these are really cool and can have you carving right through the waves of enemies. From fire arrows to a barrage of them shooting simultaneously, these can really help you rack up the coins.
You'll find great satisfaction in blowing out these groups with your new firepower. But, you have to make sure to pay close attention because remember, you're moving autonomously, so it can be easy to miss certain items that fall.
So be on high alert, because these power-ups can save you, aside from racking up more coins. So they're basically as defensive as they are offensive. Take advantage of what you're given, and get out there and dominate.