I tried to get Tina once but gave up in favor of getting Nanami, and My priority is get Koharu.
All you need (if you play the game) is to buy Paid V-Stones (worth $36 Singapore Dollars/$27 US Dollars for 5,000 Paid V-Stones) and Luck. But to increase your chance is to have 100,000 Paid V-Stones...