The fastest way to level up your characters rather than going stage by stage and killing enemy units.
1) Having two MS with over 150 ATK, prefarably one with 8x missles.
2) 2 warships with 8x missles weapon, eg Minerva, Archangel etc. Equip High Power Battery as an extra.
3) Ex-Stage 2.
In the first turn, move both of your warships to the front. Send out 1 of your MS to deal with the 6 grunts in the middle and finish them off, after that send the MS back to the warship. Another MS is send out to deal with Nataku, note that this MS should not be the Master unit of any warship as it will trigger the war break if you kill the Nataku, just kill the Nataku and send the MS back to the warship. Phase change, the 12 left over grunts will not take a move so don't worry. In the second turn, move both of your warships to the front of their respective side, facing the grunts on both sides. Phase change, those grunts will start to attack the warships, simply finish them off with your warships by using the 8x missles weapon, make sure that each one of your warship finish off exactly 6 units. End.
1) All of your warships crews receive tons of experience points.
2) All of your warships crews and the MS's pilot that destroy off the 6 grunts in the middle receive 100+ ACE points.
1) Equip both warships with Observation Pods for faster exp gain.
2) Warship's captain with Diligent skill does not affect the exp gain for the other warship crews.