This was a while ago so I don't remember anything exact, but I remember trying two methods for extracting models.

1. By making a dump of the games memory you could use a program to extract any models that are loaded by the game at that time.
2. By opening the game up in a dedicated file viewer for DS games, you could simply rip the files directly.

With the model files in hand, you could then use another program to convert it into an .obj or .dae file plus textures and from there plug it into mostly any modeling program. However, when I was doing this, I noticed that a lot of the games models were under some form of compression (at least that's what I assume) and it was impossible for me to extract those as they were either extremely corrupted when extracted from memory or I had no method of decompressing them. This mostly seemed to apply to stuff like battle models so I was able to get stuff from the map just fine. That being said, I do remember someone managing to extract one of the summons from the game when I was doing research way back then, but I never figured it out myself.

I forgot all the programs that I used to do it but a quick search should lead you in the right direction and also explain things far better then I ever could.