The best place that I have found to do this is in Coral Lakes West, Lockdown with the Silvertail Spark / Radiance dragoon. Bring an $10k+ wingman along with you that has the light attribute so you have two dragons firing at the same time making it easier to take out all the mobs. Your main skill should be enough but you can always use a light based secondary skill to help you kill off those harder mobs.
There are five waves that you need to clear. The goal is to clear all five waves with "S" rank however so long as you "S" rank four of them you can get one "A" rank and you will still get an "S" rank for the mission.
Mission Start: Move your wingman up to the front by pushing up on the D-pad.
Wave 1: Dodge! All you have to do is take no damage. Make sure you barrel roll or if you have a deflector skill then that's even better! You can fight to reduce the number of mobs but I usually like to use the wingman super attack to clear out the first run of mobs. The second run are farther away and easier to kill then dodge the attacks.
Wave 2: Use your primary attack to kill all the small mobs and switch to your bigger attack to take out the mobs that are shooting at you. When you pass the pillar where the mobs spawned send your wingman to the back by pressing down on the D-pad. This will place them in a better spot to attack.
Wave 3: Move your wingman back to the front. Kill all the mobs you see. When you pass through a tunnel will be three mobs will fly at you in the upper left part of the screen. Kill them quickly before they pass!
Wave 4: The mobs start from the pillar on your left and then make your way to the right. When you get close to the second pillar move your wingman to the back to help off the little mobs. When the four bigger mobs spawn move your wingman back to the front.
Wave 5: There will sometimes be rare enemies to the bottom right hand side. Make sure you fly to the upper right to avoid taking damage. From this point focus on dodging attacks. When you circle around a pillar with a big hole in it, I like to use my second wingman super attack when the first rush of mobs shows up. Lets me clear them out quickly and without worry about having to dodge. You can use the attack right when they show up. The second rush of mobs can be taken out easily without much fire. You'll fly through two saddles and come around to the final assault. As soon as I fly left around the second turn I use my wingman attack to kill everything. You may need a barrel roll or deflector skill to make sure you are not hit.
See my video below for an example.