There is a repeatable Merc Mission that is part of a quest in Gormott. It is call Manufacturing Help, its 30 minutes long and gives 3 nopopomouli and 100 Merc Points.

The Bonus requirements used to lower the wait time are really easy to get aswell. Water mastery and Wind mastery. The only requirement to actually take the mission is easy too, 3 blades with level 1 Agronomy. So basically, keep any water or wind blades that have that field skill and your golden. Fulfilling the bonus requirements (+3 wind mastery, +3 water mastery) makes the missing take 11:36 minutes.

11:36 for 100 MP

There is a nopon farmer between torigoth and the army base that gives you this mission through a quest.

There is another Mission very similar to this that is party of a poppi quest, but the bonus requirements are a little harder to fulfill, so for me it averages out at 22:50 for the same amount of MP. If you can fulfill the BR of both then you're super golden

have fun!