Warning: this game never ends. I just finished wave 45, and it's exactly the same as wave 44, 43, 42, etc.:no new enemy, no increase in enemy density, etc. I'd say play this game until you get the last weapon, play a round, then call it quits.
In the beginning campaigns, the heavy cannon is the only viable primary weapon (you can use a shield gun to help protect it and time guns as a back-up). In the middle and later campaigns, by far the easiest way to win is to buy a combination of time guns and remote control missile guys. You freeze time, continue to shoot remote control missiles, and when any enemy comes on screen you meet them with an armada of missiles. For both time guns and remote control missile turrets, upgrade the reload time first, then when that's maxed upgrade the other stat (not health...never upgrade health, it's usually useless.). This is completely cheap and makes the game almost boring because midway through the campaigns you'll be so dominant that you might not take any damage at all each round.
This game needs a run button, a better minimap (so can you see where each path connects to), and maybe a zoom out button to give you a better view of each particular section of the map. There's too much pointless moving around.
I haven't gotten a card in the last 10+ wins. That's well over 5 hours of gameplay (considering that I don't win every game). I am at a distinct disadvantage compared to people who can load up their characters with items and choose which characters to include. I think this is a huge flaw in the game. I think you should automatically get a card for every, say, 7 or 10 completed games, regardless of if you win or lose.
This game is ridiculously hard on "casual" mode, with "no hard waves" selected and no too-high weapons randomly found. Maybe there should be an "easy" mode. The current "casual" mode could be "Hard," and "Hardcore" could be "Insane." A lot of weapons are fairly useless, and it's just a matter of luck whether or not you get good weapons. If you don't get them, you're out of luck.