The power curve is way off. I finished the game on 'normal', and began hard. My weapons are now so powerful, on most missions the enemy never gets a shot off. I just beat the final boss and my HP dropped maybe 5% before he was killed.
penguin AI is horrible - they get stuck in corners, they oscillate between two targets and reach neither, and ignore nearby targets to pursue ones on the other side of the screen. Game rapidly becomes a grind. buy upgrades, start level, collect money, repeat. Can't finish a level? just play it 5 more times to get more money and upgrade. I completed the entire game and had no penguin fully upgraded, nor any of the 'powers'. Bombs are practically useless as they can only hit one or two enemies at most, and on higher levels since we zoom out it gets harder and harder to even hit anything.
I paused, surrendered, then started a new battle. All the characters from the last battle were still there, including my hero. wash, rinse, repeat, and now I have 4-5 heroes of the same type on the screen.
 "Home"){kind=icon}
 "Sugar, sugar 3"){kind=icon}
 "The Awakening RPG"){kind=icon}
 "Sky Quest"){kind=icon}
 "Soccer Physics"){kind=icon}
 "Dynetzzle"){kind=icon}
 "Reactance 2"){kind=icon}
 "Penguin Overlords"){kind=icon}
 "Epic War 4"){kind=icon}