Just beat all levels on epic. Here is how: Complete the game on normal mode until u reach the challenges. Use "Stop the big bang" to grind money. Max your army, your mana pool and invulnerability skill. Hit Invul when revenge waves come.
On later lvls wait for your whole army to spawn and until for your mana pool to max at 400/400, then run in and stay invul, refreshing it every 40-50ticks of your mana pool. If necessary use crit skill for support. This strat works on every level (even goblins, angel or baal) except for the very last one on epic mode (even thou the strat there is only slightly differend...). Ignore Titans, they are to slow. Just pop units and burn stuff down invul as fast as you can. If there is a mob you cant kill ignore him, as taking down the castle will do the trick aswell.
I missed the 20th treasure by 2 weeks because the last lvls are disproportionately hard, long and annoying. The game is not bad but far to monotonously to make up for the final frustration. Thus 2/5. Shame actually... its a nice game but gets wrecked by horrible balance.
 "Feed the King"){kind=icon}
 "Epic War 4"){kind=icon}
 "Viricide"){kind=icon}
 "My Pet Protector 2"){kind=icon}
 "Shore Siege!"){kind=icon}
 "Tank 2008"){kind=icon}