Spend those gems that you earn on various perks, which are in the green menu of the store (the star icon menu). Make it Rain is one of the best ones to use, as when you use it at the right time it can send your fortunes soaring sky high. Get a ton of money, then go level up your warrior comrades and let them take out a number of stages for you all on their own.
Other perks include Doom, which takes away the enemy’s hit points by 50%, and Power of Holding, which taps 30 times per second for 120 seconds whenever you hold down on the screen. Sometimes you’ll earn the tapping one randomly for no reason at all; when you do, immediately upgrade your tapping power and then use it to blow through enemies as quickly as possible before it runs out.
When you upgrade your warriors, they will have unlockable skills at various levels (10, 25, 50, 100, 200, 400, 800). These will typically cause the damage done to increase exponentially; some skills will increase ALL damage done by a certain percentage, by any hero. Go back later on in the game and level the crap out of your cheaper heroes to earn these skills.
Most of the skills, though, will increase damage done by that specific hero; the level 800 skill will increase the damage by an insane 10000%. Other skills will add a higher percentage multiplier to the amount of gold that is dropped by every enemy. Pile these up by earning these skills from multiple heroes, starting with the cheapest first.
Tap on the gold in the upper left corner of the screen in order to earn a gigantic bonus. This is gold that builds up every time that you go offline, and when you come back it shows up right at the top left corner of the screen. The more enemies that you kill while you are online, and the more gold that you earn, the more gold you end up earning when you do come back online.
There is a second reason to tap on the Angels that appear randomly toward the top of the screen as well. When you tap them, usually you will end up earning a treasure chest full of gold. However, every so often, instead of coins coming down, one or more diamonds will come down instead. This adds extra incentive to keep tapping the angels.