You can unlock the Auto battle option for a particular stage by first earning an "A" or "S" rank on it. level, which is done by completing the level under certain conditions. The following four conditions will determine your rank. Complete all four to earn an "S" rank or three of them for an "A" rank.
Complete the level normally.
Complete the level solo.
Complete the level under a particular time limit.
Complete the level without taking a certain amount of damage.
God of Iron bonuses
You will get three free offerings for the God of Iron daily. This will be a uncommon, rare, or epic item. If you offer to him enough times you will automatically receive a free gift. Otherwise, The God of Gold requires gems, the game's premium currency.
Launcher skill
One of your starting skills can launch enemies into the air. Use it as often as possible to make enemies defenseless while in the air and execute combos on them. The launcher skills usually have short cooldowns allowing them to be used constantly.
More Gold
On your character, you can forever update each slot of gear to get a little little bit of gold. The game does not describe the system but they are basically like upgrades that are passive. Gold is just required by the initial few levels, so update them as far as they are able to go. The bonuses will accumulate over time as well as your character can be rather powerful!
Selling Items
For some extra gold, try selling your old equipment. Don't sell materials though, you'll need them later down the line. Also, once you're high enough level, you can choose to break down equipment for upgrade materials. You miss out on the gold but you can get some rare materials back.
Upgrades
Spend gold to upgrade the slots of your character's equipment. They act as passive upgrades to your character. After you are at a high enough level, you can break down equipment to get upgrade materials.
Auto battle...