Gaki, Datsu-ba, and Oni are all in Ueno. To get to Ueno, first you have to go to Shinjuku, then take the middle right exit to Ichigaya, then keep going right and you'll get to Ueno. The Gaki are right where you enter, the Oni are more towards the top. Not sure where the mop things are.
I'm making a support character too. The main thing you want to do is to go for the chain expertise Enhancing, which gives you really powerful buffing spells. You need curative magic to class 2, support to class 2, and bless at class 1. You can get bless from a quest in shinjuku, I believe you need a faded garnet earring to activate it, which comes from the blue plasmas in shibuya.
Also you might want to get the Curse of the Wretched chain expertise, which gives you really good debuff skills. For that, you need destruction magic at class 1, curse at class 2, bless at class 1, and magic control at class 1. The good spell from that chain expertise that everyone thinks is overpowered is erosion hex, which you get at class 1.
Also, if you're making a support character, you want to put most of your attribute points into INT, since you need it at lv99 eventually. You can also put some into luck and vitality.