Cleric/Priest:
30-40: Wild Boars, Horned Mushrooms, Undead Mushrooms, Bears in Ludi if you get bored. The whole point of these levels is to get your heal to an acceptable level so you can move on to heal-weak monsters.
40-50: Heal should be maxed, so you can start the long, grueling process of blowing up heal weak monsters. Zombie Lupins, Chronos, Platoon Chronos, Jr. Wraiths are good. Use Platoons if you're short on cash (60% glove attack and esther shields)
50-60: Zombie Lupins are still good xp, as are Platoons. You can move up to big Wraiths in the subway (though I never really trained here because of all the hackers before) or Master Chronos. You can go to Zombies as early as 50, but don't. You'll get run out of your map and you've got a LONG run of Zombies ahead of you. Put off going there as long as possible.
60-70: Zombies. Lots and lots of Zombies. If you get bored of Zombies, Master Chronos, Wraiths, and Zombie Lupins are still possibilities. As you near 70, you can also try Soul Teddies as a change of pace, but Zombies are still faster overall.
70-80: You guessed it: more Zombies (at least for the early 70s). Soul Teddies in Ludi can be a nice change of pace, and, as your HS develops, Platoon/Master chronos parties aren't all that bad (you should be 1-2 hitting them with heal now, so you'll slaughter them fast -- just make sure the person(s) you party with can do the same -- I preferred high 60 bowman/xbowman for their nice AOEs). Around the mid-late 70s, if you throw a few points in Elemental Resistance and can withstand a hit at Death Teddies without magic guard on, you can get better xp there than from zombies (just beware of lag).