You can only get 455 diamonds from the in-game trophies, and the cheapest stuff needs 250, but the stuff above that is 500+ so you can only get the cheapest without paying. Don't bother chasing the other diamonds.
To level up your skills, you go to the skills menu, there are arrows above and below all the skills. Use those when you have skill points.
I missed that at first.
Happily, you can continue if you get a game over.
Also, if you are in a "continue" level, don't kill anything, they will gather in a group following you, and no new ones will appear.
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