@Krakenos, the trick to 50k loot is basically large combo's, a lot of training and a little bit of luck. I believe you can get 50k soulons when you hit 4k distance if you get a perfect combo every level. Use throwables if you're about to lose your combo to hit enemies off-screen.
From what I read, most people got the hard badge using a healing-based setup. That didn't really work for me: the leader was too fragile and heavy waves would pretty much thin out my ranks. So I developed a different setup, which actually got me 3301m first try. Perks: Fireproof spray, Armor, Energy Shield and M.R.E. Pack (last perk). Skill: Napalm wave. This setup focuses on preventing death altogether rather than healing damage. These perks make your allies nearly invincible. There's no need for healing perks since you get enough med kits along the way anyway. Only recruit marksmen (enemy soldiers) and rocketeers (buildings), the rest are useless. Use your skill when there's like 3/4 buildings and a helicopter, in order to prevent damage and come out safely. Good luck!
Convincing players to invest money into your game is done by creating an awesome game that becomes even more awesome by making a one-time payment, unlocking many new features and small bonuses. It is not done by inventing systems that function as a nuisance/hindrance for players until they give you all your money.
@lolomator: Of course it doesn't rewind the timer when you respawn at a checkpoint. That's why these games are hardcore. Those who can speed through the whole level without failure are the ones who are awarded platinum medals.