I have idea about big bang. Use Popo the epic dwarf and hit his spiky ball to the middle of the bomb, when the bomb explodes use shield skill to protect your units and then destroy the castle. remember to protect the popo and you ranged units so they can make max damage to the bomb
If you want to earn A LOT of EXP do this: 1- Maximize your arrows and your trap; 2- Pick any level (for best results pick level 14. Don't pick level 15 because you have to face a very hard boss); 3- Hold A,S,D and F and then click outside the game (this will make you autofire all 4 arrows); 4- Aim the arrows so that they will land a bit farther than the middle of the arena; 5- As you gain mana start increasing your mana pool; 6- As soon as you get 1000 mana build the trap and aim the arrows at the trap; 7- Continue to increase the mana pool until you get 9999 max mana 8- Go watch a movie or something like that; When you get the exp you want press A,S,D and F to stop the arrows and start pressing all unit creation buttons and let them finish the enemy castle. Rate this comment up to help the people who need exp.