The biggest problem I see
The worst thing about this game was the time it took to kill bosses. I beat the whole game on normal and it took be about an hour to kill the volcano king lightning boss at level 23. Unless you want me to grind it out, that it way too long (I presume to beat it on challenge a lot of grinding will take place). Obviously if i can survive 30 minutes with a boss and didn't use an item, then you will kill him eventually. You should decrease the hp of the bosses by half. Note this WOULD NOT decrease the difficulty that much because if you can beat half of a boss (without items that is...) then you can beat the whole boss... it's just a time issue. I liked the signets in general, but there was no reward for winning... you couldn't by anything cool but a few lame signets with the money...
P.S. What are the random drop signets i want to know if i got them all
Strategy for the game here: To be good at the end game, get the healing upgrade signet, coward, and the spread damage. Then give everyone the ability to heal. In general this will get you through the last bosses don't forget to get everyone the hp and mp up skills and make sure someone has healing aura.