There are only 3 challenges you can do with more than 10 malus, they are watering, archer, and hoeing in this order (max -12, max -16 if very very lucky, max -18 with a good map), don't bother about the rest.
For the watering challenge just do it like this:
Nope try to send an email to the support.
forums.indiegala.com
No ok honestly I don't even know if this works lol, but for watering the only tip I can give you it's just to master it. I mostly do it right left x4 and down, and I can do it with -12 seconds only if I do it with no hesitation at max speed and no errors. That's why I prefer doing it at -8 malus, since you can at least make 1/2 errors and still win.
For the hoeing challenge there was a good video posted on the thread I linked before but I can't find it, and the other videos you can found online suck, so I will just give you some tips.
Don't backtrack, don't do only straight lines, hoe everything around in your path and don't go there anymore. Every second you lose from walking from a side to the other without hoeing are at least points you don't score. I could try to record it tomorrow and post it on the feedback thread if you want.