TIP: there is a maximum of guys that can spawn at one time... so take your time and kill the archers first. for the swordman you can run around in circles and take them out 3-4 at a time and wait that they all spawn so that you don't get too many archers at once.
"number of shots" is a good upgrade; more bullets + more spread = more killing (combos very well with piercing); "bullet power" is a good upgrade, just 'cause (and combos well with rapid fire). "bullet frequency" is the best IMHO, more bullets more often. "bullet speed" is great too in particular if you are dodging a lot as they close in faster (combos well with homing). I only spend my precious upgrade points on those first; and maxing those 4 areas make level 50 much easier.
1) buy tons of heals early;
2) use boss levels to make easy money (do not kill the boss; farm the minions);
3) if they don't shoot at you or move too fast, use melee (don't waste a good arrow);
4) reload from a save to complete achievements (buy thingamagic X);
5) use decor to your advantage;
6) don't spend your upgrade points on early gear;
you can make easy kills on the 3rd boss using an axe.
in order to get the badge here are 2 tips: 1) you can't do a crush unless you're farting stars. 2) to get speed you need either the choco bars or use reaction landing on small asteroids and keep moving.
Please don't be offended but I sincerely hate games where you have to [stay away from the game] for you to be able to [keep playing the game]. The most frustrating and anxiety-building "gaming" experience. I would much rather spend 60hours trying to finish the very last level of another game with an impossible badge.
 "Forbidden Arms"){kind=icon}
 "Kids Vs. Santa"){kind=icon}
 "Champions Of Chaos 2"){kind=icon}
 "Hands of War Tower Defense"){kind=icon}
 "Valthirian Arc 2"){kind=icon}
 "Dojo of Death"){kind=icon}
 "Bitzy Blitz"){kind=icon}
 "Diepix Arena 2"){kind=icon}
 "Crystal Story II"){kind=icon}
 "Focus"){kind=icon}
 "Medieval Rampage 2"){kind=icon}
 "Escape the Red Giant"){kind=icon}
 "Super Duck Punch!"){kind=icon}
 "Cyclomaniacs Epic"){kind=icon}
 "Magi: The Fallen World"){kind=icon}
 "Mob Wars: La Cosa Nostra"){kind=icon}
 "Cursed Treasure 2"){kind=icon}
Yeah... seems like he went to the GYM from early age...