Well, this is not exacly a walkthrough (I won't tell you how to beat every single level), but it contains several tips..
Anyway, here we go.
TKK2 is a top-down action driving game, where you shoot enemies, race around tracks, upgrade your "kar", pimp your ride...and yeah, you kill bosses.
And, of course, get TEH MEDULZ.
So, let's start with the basics:
Controls: I think that you're smart enough to figure that out.
Okay, we're done.
The first levels are ridiculously easy, so l won't bother with any tips for them...
To kill the fist boss just shoot him and start running circles around him until he stops shooting, then repeat.
To kill the second boss, just shoot and avoid, and same for the third and the fourth.
As for the upgrades, early on l would recommend gun upgrades, and also the grill guard upgrade.
Later you should definitely get the rockets (damn useful) and finally the armor and exhaust pipes...the zappers are utterly useless because of their pathetic range, so don't bother with them unless you got a lot of money to waste.
To beat racing tracks just shoot whatever is in front of you and try not to get stuck (it's pretty easy) because getting un-stuck is a pain in the ass....if you're a decent player you'll get kiled very few times, if at all, during the game.

Now, to the medals part:
Klean Run: when you get some decent upgrades just play level one (do not run over them, shoot them from afar)
Krapped On: not as easy as it sounds, try to play a kill level and kill at least 80% of the enemies...when you're done, go in front of the remaining enemies and let them shoot you..when your health is almost gone, run away (or kill them).
Kataklism: easy as hell, just do what the medal description says
Krash the Kage: you'll get it by killing the second boss
Kustomised: Probably glitched, but l would adivse to carefully scan every level for extra stuff before moving on-EDIT: I PM'ed Jazza about it and he said that he would fix it as soon as he could contact his programmer-
Kash Kow: when you have 30k or more, sell all your upgrades, play a random level for the medal to activate, and then buy them back
Kill-a-k: Just play and you'll get it
Kan do: if you're not a complete failure at that game you'll get it upon completion
Kant Katch up: same as above
Kraked: beat the final boss...his health is surprisingly low, so it won't be a challenge

Okay, some last tips: if your enemy is offscreen but you know where he is, shoot away!
replay boss levels for epic money whoring
on race tracks, you don't need to kill every enemy, just shoot randomlt and run for it...
make sure you collect the health and money upgrades around the levels
use spacebar if you can't control your car well
if you really can't get the hang of the controls, play with a gamepad
also, DON'T CHEAT.You won't be able to get any more medals unless you start a new game

OK, that's it.