I've added a shield to the latest version. It only allows you to take one extra hit but I think the game feels more 'fair' now.
@ReBirth: A demo version is an interesting idea, the only problem is that people expect to get the full game for free these days - especially on Google Play. There isn't a lot of content to cut anyway, though I have considered releasing a free version without the shield. I'm not a huge fan of in-app purchases so don't really want to go down that route either.
I think my only other option is to pay for advertising and hope the boost in numbers knocks me up the chart, but that's a bit of a gamble. I don't suppose there are any other Android devs here that can advise me on advertising that works well with paid apps?
It was just a jam game anyway, I shouldn't expect too much. Onto the next one.