So if you're level 5 and you sacrifice a rat to drop back to level 4, the CP cost to go up a level decreases accordingly. I just ran some back-of-the-napkin math for a physical attacker, and I'm not impressed. Let's consider Frank in Naga City: about 1000 hit points and 90 damage. Presuming only 30% from boosters and Taurog resistances, you need to be level 6 to survive a single hit. So if you're level 6 or above one rat is worth two attacks, and if you're level 5 and below each rat is worth one attack. After running some numbers, I determined that level 2 is the most CP-efficient. Presuming unlimited rats, the ideal level to fight Frank is the 2nd. You deal 42 damage per hit (10 base attack, +4 fine sword, +2 sword, +5 skullpicker, +100% attack bonus). Returning to 2nd level costs you 40 CP, so it costs you about 1 CP to deal 1 point of damage. This means defeating Frank alone costs 1000 CP. And that presumes limitless rats, in practice you only have a limited quantity so you'll probably need to be less CP efficient than that to conserve on rats. It is impossible to beat Naga City like this.
I'm going to do some math for max-MP stacking to see if that works better, but I'm not holding my breath.
As it stands "boost to level 10 and just fight the bosses with the resources available" seems to be the most sensible course of action for vicious.
