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One way you could do this is by starting Greed Mode with Isaac and using the bomb he starts with you can blow up the Shopkeeper for a chance to get the "Head of the Keeper". (Enemies have a 5% chance to drop a Penny when hit with Isaac's tears.) If you're lucky you might get an enemy that spawns enemies (which is pretty common in greed mode), so you keep killing them until you're at 99 and spend it all. This took me about 45 minutes to do. "Head of the Keeper" has a 1% drop chance, but with enough restarts, it shouldn't take long to drop.

This does not have much rng factor since you only need to get one item, and since you're in Greed Mode you have ways to spend the coins.