I think Odin hits all enemies, so 1) you wouldn't need Vivi's Death, and you can switch him with Eiko; and 2) you wouldn't need Quina. If this is right, I'd suggest figuring out how fast you can get Odin'd battles to go by. Then, if it's the fastest (the animation takes 30 seconds so I'd think so), you should probably switch her in as soon as its feasible for her to end the battle with one summoning. That's 99 ores, and then she needs at least level 20 ([99/2]{ores}-72{Yan's level}=23, I don't know her magic stat). You know how difficult it is to get lucky, and how much % you need to realistically get the OHKO.
So, keeping the battles up as you do now, until you hit 99 ores, you need to fight at least one battle (without virus'd or dead people) to get Garnet to level 24. (exp needed is
30,786, 3*Yan gives 32k to 4 people). The first party to get leveled should beat this asap then. There are two spots taken by Zidane and Garnet, two spots are left. Freya's Freeze, Amarant's Death and Quina's Sleep are the important options (Vivi's Death has no chance of hitting yet). Quina is the safe option, but Freya and Amarant might be faster. I see in the
Monster List Yans can cast snort, which could be useful: more experience to Garnet after the battle, I think.
If this works, it's Zidane and Garnet all the way, and switching members three times. Use Virus Powder on Zidane and Garnet to regulate which members get faster experience and thus less AP.
Alternatively, if Freya Freezing/Amarant Deathing/Steiner Heating/Zidane shattering is the fastest, it might be worth it to safe Freya's and Amarant's leveling for last, and have a slower start with Garnet.