This one's exactly as it says and can be obtained in any of the Bard's Tale stories.
Once you've got a bit of coin, you can always spend some time grinding this out in the blackjack tables in any of the cities (including Skara Brae in BT1), although you're restricted by how much you can bet, so it takes a bit of time. (Still, it's easy enough to take advantage of a save/load if things aren't going your way.)
It shouldn't be necessary, though, as you'll get 100,000 coins naturally just by playing through the games. Notably, finishing Bard's Tale I nets you 300,000 coins from the king, so if you haven't picked this up before then, that'll do it.
Edit: As noted by NafrayuNox:
In the catacombs lvl 1, on the square N 12 E 3 there is a fight against 66 Skeletons. You can easily kill them in one turn with magic AoE attacks. After opening the chest I usually get between 7k and 9k. Go back up the stairs and back in the catacombs and you van farm this achievement in about 20 minutes.
FWIW, this is also a nice place to do some levelling, once your casters are suitable size to kill them. Unlike many of the other fights like this in the remastered Bard's Tale Trilogy (which can only be done once), this one's there every time.