Level 10 survival took some doing but I managed to beat it. Manage your towers wisely, and remember you can sell them once they've served their purpose if you need the cash. There is a hut in the center of the map (and on some other maps) that deals decent area damage for a small fee and can get you out of some tight situations.
@ssgs re: rocket skill. yes you get random extra rockets when the skill is maxed, but it's oh so useful when the screen is full of bad guys :)
Although I complained that this game was too difficult (twice) Let's get one thing straight - this is NOT the sort of game you can expect to complete in a single afternoon - it requires a certain amount of dedication that is usually only found in retail games.
Accept that you're not going to beat this game quickly. Take the time to look at the achievements, they'll teach you styles of game play that will help you in the long run.
Do each level in survival mode, it will teach you how to deal with limited resources, and gain you 3 more stars per level. You will have to restart later levels multiple times before you even begin to make progress on them.
Reset and customise your bonuses before you play each level, sometimes this is a necessity. If you're playing a level that is going to need lots of heavy machine guns put your stars into that and treat everything else as secondary. This is more important in the survival modes, where certain weapons and power ups are unavailable.
Great potential, and the first 5 levels are great, after that the difficulty ramps up to near-impossible, and the difficulty setting seems to do absolutely nothing to affect gameplay (other than award you less stars).
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100%!!!