I don't quite know how to answer this question, but from my experience, switching to gathering "Undead" mobs in Mooshu and then in Dragonspyre raised my levels pretty quickly. (I was after an "undead fairy" inside a Dragonspyre instance, the only collectible fairy so far.)
After that, I started collecting "Wyrms" in Dragonspyre and then dragon bosses Avalon. By the time I was satisfied, my wizard was around level 25 in Monstrology.
So basically, to level up you should push yourself into the middle worlds as soon as your Monstrology rank goes up high enough to match those mobs and bosses. Keep going higher and gather rank first, then go back and collect lower level stuff at your leisure.
If you want an exact number, it's like, 6 Monstrology levels above the enemy's rank. I remember hitting Level 9 and no longer being able to farm Nightshade for XP.
I've been doing Monstrology from a Pokemon standpoint (Gotta Catch 'Em All). Not from a "Must Level Up" strategy and noticed as I was working my way through Grizzleheim that some of the lower rank mobs and bosses were no longer giving me Monstrology XP.
So, anything lower than six levels down from my current level will add to my collection but not give XP anymore. Good to know.
80
Alia Misthaven