I would add that your #s 3 and 4 are grammatically incorrect. Don't use them. Jessica gave a succinct and perfect answer. You should use those suggestions. Good Luck to you!

- We can only try to waste as little time as possible. ok > Normally used when you know how long it will take and try to get it done faster by not wasting any time - We can only try to waste less time. ok > Normally used when you know you have wasted time doing something before and try not to now. - We can only try to do not waste more time. X We can only try not to waste more time > You have wasted time and try not to waste more. - We can only try to not waste time. X We can only try not to waste time > Before you started doing it , you said you would try your best not to waste time so that it could be finished earlier

Mm... I think the best way to phrase this may be, "We need to waste as little time as possible," or even, "We will try to waste as little time as possible." Something along those lines would work. Hope this helps!