Although this game is really easy and straightforward, there are some tips I can give everyone which will help you throughout the game (not to mention that I had to find them out the hard way)
First off, you will concentrate on upgrading each character at the shop to their maximum. There are also kill-related achievements for the different types of enemies AND the different playable characters. So I recommend upgrading one character at a time, upgrading the health first, then the second bar and finally the last one. Go for the kills and as soon as you reach the highest one (which is normally around 250) switch characters and do the same for that one.
There are some parts in the game where infinite enemies will spawn and you have to activate a golden statue to progress (normally these parts include a boss fight). If you want to you can grind kills in these parts since you will stay in "god-mode" until you activate the golden rubble. These parts are also great to grind studs since you will need a lot of them and earning the 5 million mark will take you more than 2 playthroughs.
You can use whichever character you feel most comfortable with, but as a mater of fact, the black mask is by far the strongest one. As soon as there are big droves of enemies gathered around, use the attack and about 90% of the enemies in close vicinity will die instantly. The bigger variants need about 2 shots which is, in comparison to each other weapon, a lot of damage.
Collect every chunk of studs you can get! As long as you are in god-mode you will get no damage AND and increase in damage output so use that to your advantage, clearing every area of enemies BEFORE you use the golden pile which will automatically turn the god-mode off. In this mode it doesnt mater which character you use, although it is again the black character which has the highest damage output.
There are 2 categories which your characters are in : heavy and light. Light are blue, green and red therefore having less damage output, high shooting frequency, but more movement speed. Heavy are white, yellow and black therefore having high damage outputs, slow shooting frequencies, but slower movement speed. Using the light classes for fast running around and the slow ones for killing everything in their path is the answer and should be kept in mind. However the best character in the game is the one you will unlock after the boss fight since it has both, strong attacks and fast movement speed.
Unlocking the things at the shop will add to the completion, also collecting everything and getting each achievement. Try to buy the "50% off" item from the shop first, which will permanently decrease the cost of shop items.
The bonus areas are located on the playfields after the second beach site in a small tunnel and the collectibles you got so far are at the starting area upstairs through the doors.
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