Are you annoyed of receiving not much power?
The Actual Guide
In my opinion the fastest way is doing the following mission:
- Urgent: Roboresurrection (Level 25)
The only requirement is that you are already in the region of Cauthess and that you already unlocked this mission. This mission provides you 6.000 power and with a random group you will pass this mission with an eye-blink (2-3 minutes).
The only disadvantage are sometimes the long loading times.
Does This Really Work?
Yes, of course. I already used this method on the PS4 and because the Boss of the mission (Mech) is already available when the mission starts it is a good way to grind for power in the early hours of Final Fantasy XV: Comrades.