For the MC, the "Special Training" map is by far the best map for leveling up. It costs 1000g to enter, but you get lots of EXP for choosing the submissive options.
For other characters, killing enemies gives more EXP than just damaging them, so maps containing lots of weaker enemies (including additional spawns) are usually better, so long as you stay out of range of the boss until the end (and can then beat the boss).
A character with 30 Def holding either Charlotte's Shoes or Anabelle's Panties will have 33 Def, enough to completely block the Queen's weapon.
You can get Annabelle's Panties in the same location as the Blackmail Material, and Charlotte's Shoes come from beating "Charlotte's Web" (you can get multiple). The Royal Bra also works for this, so long as it isn't the MC holding it.