I complete my epic boss levels in a single run. Do not just focus on upgrading/ making high level gems.
Step 1: Stockpile on many high grade( well perhaps grade 3,4,5 gems) till the epic boss appears.
Step 2: Create a few water trenches around the monsters' den.
Step 3: Put a few high grade gems( grade 5/6 or maybe even a 7 if your mana is high enough) in the towers closer to the den.
Step 4: When the boss appears give him everything you have from your inventory( all the gems you have been saving) in the from of gem bombs.
For a more comprehensive guide read my guide!