Pangeon (UP)

To unlock the achievement you must have 250 gold ON HAND in your possession.

To unlock this fairly quickly, create a new game on Normal mode using the Thief player class. I dumped my starting skill points half and half into Health and Luck. Play through the entirety of Level 1-1, looting every chest and conserving your items. Once you've made it to Level 1-2, clear out the level up until the point where you fall down into the dungeon section (The door at the bottom of a staircase guarded by a single bat leads to this point of no return). Also, don't forget to pick the lock on the door at the top of the staircase and obtain the loot within said room and to loot the chest underneath the staircase.

Once you've looted everything, return to the vendor at the start of the level and sell everything you own. Strength Potions easily sell for the most gold and you should have at least one of these on hand. If you've looted everything and been fairly lucky with drops, you should manage to get well over 250 gold at this point (I had 303 here).