This is a pain, especially on harder monsters. I'll give you some hints on the "worst of the worst":
Atlas: His whole line is a pain, since his Wanted can OHKO you and you really can't afford to lose any Leaves along the way. Have a Lv. 99 party, and play extremely defensive using Cold Fusion/Inferno Slash on the whole grind. If you're one of those who doesn't get a lot of consecutive time, plan for at least a one or two hour go at it.
Pazuzu: 4 endure rings is a must. Thwack will ruin your run at them otherwise. They also don't stun and are rather fast, so be ready.
Roseguardin: I hate how hard these things hit, and their shield! (I think their shield on Wanted doesn't break period, even with abilities). High level, circle around, suggest Alena (if you got her). If not, use the Hero, Cold Fusion/Inferno slash defense, AND BE CAREFUL!
Crimson Killing Machine: Probably the hardest one to get. They are immune to any robot zapping strategy and do boatloads of damage even in regular form. Worse, there are Wight Kings and other nasties in the area, so you can lose Leaves in your run rather rapidly. Play defensive throughout, Cold Fusion/Inferno Slash EVERYTHING, and don't lose Leaves early. IF you do, just go back to Accordia and have another round.