There are several ways that you can get more gems and coins, you can go to the Tavern and login with your Glu game account or Facebook or Iphone Game Centre and invite friends. Watch daily videos at the shop, take the survey, subscribe the sponsors, following Glu in Twitter or liking Glu in your facebook.
A good strategy is to always have a Speed Weapon and Big Weapon as your alternate weapon. Speed Weapon are effective against speedy type monsters while the Big Weapon is good for groups and hard to kill monsters.
Complete Stage 1 up to Level 30 and then quit the game. Now repeat the game starting at Level 27 and every time you reach one level up quit the game and store your souls in the Forge. Remember to open the treasure chest before leaving the portal.
Quit the game when you complete a level up and go to the Forge and convert the soul to coins. A good strategy is to always make sure you have souls to convert so you can gain extra points from each of the chambers.