Dave Pro recently took the time to research the points at which your characters will gain only 1 experience for defeating the various monsters. The levels below indicate the level at which this happens (a + indicates the level is after promotion). The following are true regardless of any levels gained before promotion. So in theory if you level up to 40 before promotion, you should continue to gain reasonable amounts of experience on monsters that would only give you 1exp after promo.

Gizmo 7 Ooze 7 Huge Rat 7 Huge Bat 7 Galam Soldier 9 Galam Archer 10 Galam Mage 10 Galam Knight 10 Dark Cleric 10 Green Ooze 12 Goblin 13 Dark Smoke 14 Hunter Goblin 14 Witch 14 Vampire Bat 14 Dark Dwarf 14 Evil Monk 15 Hobgoblin 16 Death Archer 17 Evil Cloud 17 Zombie 18 Golem 19 Black monk 21 Master Mage 21

Gargoyle 2+ Soulsower 3+ Arrow Launcher 4+ Orc 4+ Dark Madam 4+ High Priest 4+ Harpy 4+ Skeleton 8+ Bubbling Ooze 8+ Rat 9+ Lesser Demon 9+ Dark Bishop 10+ Dark Soldier 10+ Dark Sniper 11+ Lizardman 11+ Worm 12+ Wizard 13+ Wyvern 13+ Orc Lord 14+ Dark knight 14+ Harpy Queen 14+ Master Monk 15+ Devil Soldier 16+ Ceberus 17+

Pegasus Knight 17+ Necromancer 17+ Bow Master 17+ Mud Man 18+ Shaman 19+ Dragonewt 19+ Purple Worm 19+ Griffin 19+ Bow Rider 20+ Executioner 20+ Chaos Wizard 23+ Mist Demon 23+ Prism Flower 27+ Hell Hound 22+ Minotaur 22+ Evil Bishop 25+ Cyclops 23+ Dark Gunner 25+ White Dragon 25+ Hydra 26+ Demon 27+ Chaos Warrior 27+ Reaper 28+ Chaos Dragon 28+

Horseman 28+ Demon Master 29+ Evil Beast 29+ Blue Shaman 29+ Devil Griffin 29+ Pyrohydra 31+ Arch Demon 31+ Zeon Guard 32+

Many thanks to Dave Pro for allowing me to publish his list here!