User blog:SlashZero/Winter 2016 E-2 (Hard) Maruyu Leveling

What
The first two nodes of E-2 cannot damage submarines. The first node is sub-only, and therefore consumes 0 ammo. The second battle is an airstrike, which gives ranking based on damage taking, with no damage taken resulting in S rank. It also takes 1 bar of fuel and 2 bars of ammo. This allows:
 * Positive morale gain for submarines over two battles
 * 50% win rate
 * Average usage of 2.5 bars of fuel and 2 bars of ammo (for Maruyu) per sortie.  Other submarines will use 1 bar of ammo per sortie.
 * Around 800-1000 exp gained per run
 * No chance of taking damage, unlike 5-3 grinding

How
Repeat as many times as you like for exp.
 * 1) Make a fleet of a solo submarine
 * 2) Deploy to E-2 on the hard(甲) difficulty
 * 3) Retreat after the second node
 * 4) Don't resupply
 * 5) Repeat 4 times
 * 6) Resupply

For sparkling, three runs should give 85 morale, assuming a start at 49. However, you can't save the 2 bar of fuel from starting on 1 bar.