Pretty good game but it does freeze up from time to time, so minus half a star for that. It's particularly annoying if you were having an extremely good run, then the game freezes (well, everything still moves but you can't do a turn and neither does the opponent). You just have to refresh the page and forfeit that run.
Hardest medal to get is definitely Nerd, though it's only a 10 points medal. You also have to plan out what to do to some extent, because what gives you the most money in a run isn't necessarily the same as what gives you the most score and vica-versa.
4 or more in a row is what gives extra turns, so if you keep getting 4 or more in a row, the opponent just never gets a turn or vica-versa.
Score is actually tricky because it relies on several factors, a major one of which is your max life and life remaining, but also highest combo. Well if you're just way overpowered you won't get very high combos because you'll just kill the enemy too quickly. But if you're way underpowered you might lose.
I currently have the highest score, and I didn't play it like this, but I hypothesize you could achieve the very highest scores (much better than mine) if you just didn't put any upgrades on attack, put all upgrades on health and shield, as this would drag out the battles longer (more max combo per stage possible without killing enemies too quickly) and also gives the highest max health possible, and just pay attention to trying to get those shields and hearts in-game.
Very cool enemies!