I am going to share with you couple demons that I think are really useful. Not only they are great help for your current level but they can be easily fuse into other, higher level useful demons, so leveling them will only help you more in the future.
I am only going to talk Law demons here so there:
Principality
level 25
Family: Divine
Close: 37 Long:37 Spell: 34 Support:43
Not only Principality has really high support, 43, but it also has a feature called "Technician" which reduce cool down time of it skills. That pretty much make Principality the fastest caster and best healer you can get at that level.
To top it, its very easy to make it into Goddess (just fuse with fairy, which are basically everywhere) which is the family of the next demon I want to talk about. If you are law and melee. this is one demon u should have, it will keep you alive.
How to make:
archangel + fire element (inexperienced jack + pixie).
P.S. Put good skills into your archangel
Sarasvati
lvl 30
F: Goddess
Close:35 Long:41 Spell:46 Support:50
I like Saravati for many reasons, her support is awesome so is her spell, along with her healing boost (50% more effective when use healing magics) makes her a good, balanced caster, offensively and defensively.
Further more, she is required to make the lowest level tri-fused law demon: thoth, level 42. So put some good skills into her, especially healing and offensive AOE.
How to make:
Principality + Gandharva = Saravati
I am going to post another topic about good law demons from lvl 31 to 50.