The Damage from Hecatomb Wave can be high.

The Slow from Demonic Howl is weaker then Haste, but still pretty large.

This spell can be safely soloed around your mid 50's. If you wish the spell when you can learn it at 47, you will need at least one High level Helper.

You will need a way to sneak and invisible to camp. Eye Drops can be an aid. Setting Chaotic Eye is Helpful.

The Best place to hunt Demons is in Xarcabard. There two good camps in the Zone. One on the ramp at H-7, and the other in the area leading up to Castle Zvahl.

The Camp at H-7 has less Demons about, but is a very safe camp, though you may need to kill an Ahirman or two to pull cleanly.

The Camp in front of Zvahl has more possible aggro on the way and in it, but more Demons to fight.

Demons will link so always pull with care.

In either case, Sneak and Invisible yourself to camp, and pull a Demon to it. If you are going solo, you'll need to go heavy with your attacks to wear the Demon's HP down quickly to at least half, before switching over to a low damage weapon, and holding on with cures as long as you can while you wait for the move. These monsters do hit hard and you want to be able to finish them fast if you need to. The Black Mage, and Dark Knights will cast Magic, so silence and Stun will help. The Summoners will have a spirit out that will attack you also. If you can pull it without its master seeing, you can easily kill it.

Having a Higher level Tank to hold the Demons will make learning the spell a lot simpler. Simply keep your Tank healed with the Occasional Wild Carrot, and feed the Demon TP with your low damage weapon (A great time to skill up club!), stun any magic the casty sorts try.

There are a lot of other Demons, all require more effort to get to and more effort to kill, with the other good low level camps, none of these are recommended.