Fastest way to do this is set the difficulty to [MEDIUM] and search for a mission objective: "Eliminate a Charger".
Bring the RPG (The reloading rocket launcher) with you.
Once you find the Charger, hit it once with the Rocket Launcher. (Try aim for the side so it breaks the armour.) Then fire a couple of shots into the weak-skin with your normal primary. The Charger will show some indication that it's weak.
The Resupply Pod can stick onto the Charger's weak-skin, however even if it does stick it can miss if the Charger is 'Charging' when the pod is dropping. Try and keep it in one spot, stand close too it and do your best to dodge. May take a couple of attempts if you get bad luck.
That's how i managed with it.