Work in progress...
EX quests are not that difficult. Just level-up your characters to 45~50 and use your friend's Thancred or hire powerful units.
Expected value of pulling Y'shtola Fragment from the Ticket Gacha is 6 (1 ticket = 200 Tomestone). Converting it into 100 Tomestone, expected value is considered as 3.
Using the summon ticket looks good option though, but it's hard to say "better" since still depending on luck.
Expected Value (Y'shtola Fragment) = 40 x 15/100 + 0 x 85/100 = 6
・Purchase: 1 Y'shtola Fragment by 100 Tomestone
・Ticket Summon: 3 Y'shtola Fragment (Expected Value) by 100 Tomestone
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Diabolos | Vision Card