QUICK TIPS:
Stomping quests - challenge boss 2 + equip pistols (easy to get 10-20 stomps per run)
Open containers quests/Kill bosses quests - challenge boss 1 (dies fast, gives 6 keys, so 5 containers per run)
Pulse gun is the best till you get Laser/Rocket launcher
Buy Infinite double jump power-up and stay off the screen all the time, falling down to get a medkit/plane/helicopter etc. Also, buy a jetpack - easy 4 km, so you'll only need to run 16km, not 20.
[+] to keep these tips up.
Let's not buy a bunch of on-sale games from Steam, let's spend $10 on a smegging flash game. Right. Every game with real-money microtransactions must be banished from my Kong.