well, I just beat her and what I did was very similar to what you describe. I didn't have the block summoning song so I held her off as I upgraded and then made some roadies. The trick I found was that Ophelia will march about with her army when they go out for an attack but will recall them immediately to defend her stage. If however they are say about to attack your stage when you begin you attack on her she has a long march ahead of her. She wont make it back before you've trashed the place.
Once you have a decent little army with some roadies send a small group of headbangers off to attack a geyser and draw her to the other side of the central battleground. After she commits herself to the defence, move the headbangers back to your stage. Ophelia will continue to press the attack and when she does, run your army down to her stage and finish the battle.