You really have to use ice. You didn't mention what kind of chiller you have but there's always a way to use icewater as your coolant. You can use gravity or a small pond pump. I like to get my lagers down to at least 65F before I pitch but even then, it goes right into the lager fridge at 53F. The last time I did a lager, I got it down to 58 before pitching and it took a good 36 hours to really take off.