I have Chichu, but I would like to be able to use Twilight Odin for fun. However, Odin's stats are beyond horrendous to start, and they grow at a snail's pace while wasting expensive monster materials.
At what level will Twilight Odin be decent enough to use even just for fun?
As MightyGeb already stated his attack speed is absolutely deplorable to the point that even if his STR and MAG could be raised up towards the 2k range he would still be inefficient to use compared to Chichu.
But you did say you wanted to build him for fun (which I understand since I did too). Most of his stat gains are from levels 80 to 99, iirc. Before then he will continue to grow at a snail's pace, but the gains you get in those levels are far better than any other monster gains at any level range in the game.