Too easy, too little levels
Pretty fun and casual game. Would be much better if there were more levels to try out every gun.
I read that the chaingun isn't really a good choice so I only bought three guns, AK47, Shotgun and Grenade launcher, all fully upgraded and all good choices.
Get AK47 after the first round, then upgrade it whenever you have the cash until its fully upgraded. Then get the shotgun and do the same. At this point you can already finish the entire game using the shotgun.
As for the grenade launcher, don't use it until it's fully upgraded, but it's a beast when it is. It works like a mortar and deals equal damage to everything within the damage radius. A fully upgraded grenade launcher can 1-hit flying penguins and those icy penguins even when they're iced up (taking less damage).
You don't even need to aim with the grenade launcher, just point it towards the middle of the road near the end of the screen and everything will vanish before they even make two steps onto the screen. Area effect doesn't work so well for aerial units though.