For anyone having trouble with I'm back, shocked? there's a really easy way to do this otherwise random and irritating challenge.
Fast travel to Three Horns - Divide, then all you need to do is go right to the group of Skags and shoot at them so they come rushing at you and when one of them is close to the Stinging Cactus, just shoot your feet with a rocket launcher to down yourself, then shoot at the cactus to get your second wind. You can do this on normal mode.
This trick was shared to me by a friend, who got it from Gothalions Prestige Guide on YouTube.
I'm not going to do a guide for the other ones since they are easy and the information is already on guides on other sites anyway.
However, if you're having problems with a challenge, just leave a comment and I'll help the best I can.