You could add something specific to that level, or just use multiple conditions that if true, will unlock the achievement.
I'd imagine if you haven't, you'd want a condition to count the amount of gold on level and set it to a variable on start of layout. You can the add multiple conditions say level = 16 and GoldVariable = X amount > unlock achievement, or unlock it in the array. Remember a condition is always an and condition unless you specify the block to be an or condition.
If your not counting levels with a global variable you could try adding a specific event sheet for this level and including your engine event sheet in this event sheet.
Hard to how your doing things without knowing how the game is coded.