According to the database, apparently after Myon is dead Youmu will use all four attacks with equal probability. However, Slash of Present targets a random slot and Slash of Eternity has back row perferrence.
Yeah, so basically what you want to do is shuffle your walls to slot 3 and 4 instead. Apparently.
Alternatively, you can try killing Youmu first. Myon doesn't go as crazy when it's left alone (the only change is that she starts using Freezing Wind, and a lot of it, but I dunno - if Patchy has 320 MND she basically takes no damage from any of its attacks, and otherwise it should be quite easier on your team since they tend to be more survivable towards magic (or in Cirno's case, CLD). I don't know how feasible is that option, though, since that means you have to weather the assaults from both of them a lot more.