Easiest way to get to 12 is to replay the earlier levels after beating the Last Relic level which is the Eternity Boss Fight. The game makes you to advance at this point anyway
To save time I would play the following levels with the following catalysts.
Downfall Well (Crystal Viper Obsidian Warastle) 2 Power
Trinity Tower 4 times - Impervious boss fight all 4 catalysts ( I tried downfall well 2 in my run-through but could easily shave 5 minutes off just using Night Blade for this one) 4 Power
Xeila Crater (Crystal Viper Obsidian Warcastle) 2 Power
Molten Castle (Crystal Viper Obsidian Warcastle) 2 Power
Leytharus Night (Obsidian Warcastle) 1 Power
Last Relic (Obsidian Warcastle) 1 Power
My full run-through of the game is below if you need to see any tricks on the boss fights.