Easy gold medal?
Solution 1:
In fact, there is.
Luckily, in order to get the gold medal, you do not need to fulfill all conditions in the same run. Thanks to that, you may try to complete a mission within the time limit, then retry the mission and attempt a longer, yet safer run.
At the end of the mission, you'll get a report saying that you did not complete all challenges and therefore only get silver. As soon as you look at the report in "Replay Mission", however, you'll see your gold medal.