Well extra stage I feel has better rates of chance/battle (could be totally wrong on that), so getting to it quickly probably helps getting them. Also opening a lot of cases can help if you get radars/bouquets. Between those 2 things I prob got most of my bouquets that way. I did not get many bouquets while climbing the tower.
If you are really at 15k currently depending on how many radars you may or may not have saved you prob should just save resources. It just depends on how important the card is to you. If you feel its worth it then can keep trying, may get lucky. As for gold, as foyn said stuff for elite guard, also if you amassed a lot of n wild cards can trade them in if you want.