Easy "Rolling in it!" trophy
Collect 100,000 gold during the campaign. This does not have to be done all at once; upgrades can be purchased. You check your progress at the level selection screen under "Campaign stats". When you have over 100,000 gold, you may need to complete another level for it to unlock. Consider finishing the campaign first to get a lot of gold. You can get up to 1,000 gold per minute by farming the Mountain level called "Statues". Collect all of the gold multipliers before killing any guards. You will get a few hundred gold from each one instead of the normal thirty to fifty.