The Binding of Isaac: Rebirth

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.

quite luck based as you need to somehow get that much money and then still have enough ways to get rid of it. if you're lucky, you can play a slot machine until it blows up and gives you the Dollar Bill, which is an immediate 99 coins. the inverted Hermit card won from the Red Redemption challenge "sells" all items in the room by converting them into coins based on the price they'd sell for in a shop. Ace of Diamonds can turn all pickups and enemies in a room into coins, so you could use that in a large room full of enemies.

Whatever way you get the money, now you have to get it back down to zero. The earlier in the run, the better, since you'll have more shops and such to hit up on the way down. You can spend it on anything, shops, donation machines, beggars, slot machines. personally i went to an arcade and played the shell beggar and slot machine until i got down to 15 cents, then spent the last of it on an item in the shop.

as soon as you hit 0, achievement unlocked

My game crashed and i had 99 pennies before it crashed, when i started it again i still had 99 pennies but the achievement was unlocked, maybe this can be replicated?

Patched!