When you see exclamation mark flash over monsters head get your heroes to use their skill and damage the monster in question. Keep in mind that your skill actually needs to damage the monster in order to interrupt them.
Keep upgrading hero stats with gold you earn from quests and chapters.
Rubies allow you to buy contracts, which allow you to recruit new heroes.
Earn rubies with the consecutive login reward bonus. Logging in on the second day will net you 100 rubies.
Fighting through Manatech Tower, Daily Dungeons, and Raids allows you to redeem 'Adventure' Quest rewards, which are worth one ruby per quest.
Enhancing hero stats and equipment each have their own 'Growth' quests worth a ruby as well.
Sending Social Points to friends gives you free rubies and winning PvP matches in the arena as well.
Some of the achievements in the Missions menu reward you with rubies.
Don't spend your rubies on stamina (you can use social points instead) or gold.
Don't buy the basic contract with rubies as you get one for free every two days.
Checking in on the 14th day gets you a 4 star contract, and the 28th day nets you a rare 5 star contract.
If you completing the 7-day consecutive logins, reward bonus grants you the 5 star card Ceris.
Completing an entire chapter will always reward you with a 3 star hero contract.
Completing the 'House Leader Training' quest line nets you a 5 star contract. Make sure to check in manually when you complete each step in the quest line, as you won't get credit if you haven't accepted the next part of the training.