WET

These aren't too difficult, you just need to know where to go and be able to shot the targets to lower your time. you may be able to miss one maybe two, but any more and you might as well restart and try again. i would say its best to give it a test run so you know the route and where the targets are. Also if you want to scrape a few seconds of the start, when the countdown starts, unplug your battery pack then plug it back in again, then when you take it off the pause menu you should be able to start as the countdown it taking place, giving you a 3 second head start.

For the mini gun challenge, just constantly fire as it doesn't overheat and make sure you make the disks your highest priority as they will get you the most points.

Also using the youtube videos for the B-line challenges are very helpful and make them easy to do first time if you just follow the route.