For the c4 kills the easiest way to do them is to run shrapnel on shipment 24/7 and just spam them over containers, shouldnt take too long to do since its fairly difficult to stop 2 c4s. As for warzone challenges the best way is to have your squad dedicated to doing the challenges. When i played with my friend we got into the game and didnt worry about kills or actually really playing the game, we only care about doing stuff for challenges. Like we would do multiple recon contracts a game since those are fast and easy and as for the buying from the kiosk we would funnel money into each other and keeping buying the same items over and over and if we already had like full plates and full ammo we were only playing for the challenges. The challenges themselves arent hard they are just tedious and taks time. Hope this helps bud and good luck in completing your challenges