It is DEFINITELY worth it to max out duel gem mastery. A duel gem in a trap can do 2 to 4 times the amount of damage than a pure gem of the same grade. This is very useful on swarm mode. Mana Gain, Forge, & Replenesh are also good to master.
I found a glitch (in this version, the "agame.com" version, and the not full version here on kong. When you start the game, you are given a mission to kill 3 slimes. Then you are given a mission to kill 10 slimes. These before the "kill 10 slimes" mission is over, you will have gotten enough experience to level up your stats. HOWEVER, when you finish killing the 10 slimes, and you go back to the town to except your reward, you will notice that you dont have the exp. points you just earned from killing the slimes (not the points earned from completeing the mission). (so if you level up and spent the 5 points on strength, then go and except your reward, you will have only 3 points in you strength catagory) WTF?!?
I believe there is a way to "carry on" with out accually clicking on "carry on". Be sure to save after every battle (i think it autosaves) and if you die, just click on "back to title menu" and reload. ta-da! continue on with out carrying on :D