So the boss sets it's level based on your starting level of the event. So the first thing I did was not do anything with the event until the last few hours to get as much progress between the start and the end of the event. I prioritized leveling girls, then spent all my money on stats, made sure my equipment was good, then applied three legendary boosters (the purple ones you acquire from some of the free orbs you get). I then auto-assigned all five teams (in retrospect I should have made the first girl in each team the stronger color but that's okay). And then I did the opposite of the conventional wisdom above. I used my WEAKEST team FIRST. I went up the ladder using the weakest teams to exhaustion, then choosing the next strongest team. I am level 234, I have no girls over lvl 500, and doing this strat I was able to beat the boss with like 2% left on my strongest team. It was really, really close, but I think if I had moved my girls around a bit, making sure the first girl in every team was the right color, I would have been slightly better off. Either way, I think this will minimize your need to spend gold.
If it's true that if you use a sufficiently strong team they won't take damage then I would amend my strategy above to using my strongest team first, until they take even a single percentage point of damage, then pull them out, going back to my weakest first strategy. The reason being the name of the game is to minimize damage, the weakest teams are more effective at lower power levels for minimizing damage in aggregate and the stronger at stronger tiers. But if a team can fight and not lose any percentage that's obviously the best call. Will experiment with the next one, but like I said I got this one without gold which was nice.