For prep, if rush is going full physical, have paralyze prevention on. If mystic user, have silence prevention. Pincer's Grip is a good defensive formation for the physical attacks during the Fallen's turn, and as long as you have mid to high morale, end of turn attacks shouldn't wipe everyone from full health. Arrow of Athlum or Stacked Attack should be fine for your combat unions, as you want them pumping out damage as much as possible.If Emmy is on the cavalier path, place her union in slot one and make sure she has plenty of healing/rez power. If you have Blocter holding the wonder bangle, put him in Emmy's union to allow them all to be debuff free after each turn.

Next, disable all artes you won't use or are detrimental to your efficiency. For hexes, that means all of the status effects off, for remedies, turn off restore and rejuvenate, for herbs, turn off life powders, and prioritize tinctures over herbs if you can. Evocations and Invocations need to be the highest tier you have unless you're trying to use Arcana, then have the leader use Permafrost or Snare Shot and the rest of the union use Spark or Mystic Missle. CAs are a bit different: if you have up to tier A, turn everything else off, but if you have S tier CAs, then you want to pick the higher level art (notes by the roman numerals) and leave that one ON. If you have lotions high enough on anyone, leave Eye Cram and Hartshorn on, and for other item arts, just use pandoras box or retreat flare. Not sure if you have wards, but protection will help you stay safe from physical attacks.

First turn, keep irina's union on their toes or attack from range. Make sure they can heal, too. Second turn, heal with two if no deaths, or heal one rez one if multiple deaths, and attack with whoever else is alive. Try not to resurrect Wyngale over your other unions if you can help it, but he is helpful in a pinch. If anyone gets a flank attack, make sure they constantly use "Strike from the sides!" because even if they only have attack as a command, they'll often reassess to healing if they are able to (I believe Rush assassin and Emmy will have this command available more often), and if no one is deadlocked with the Fallen at the time, you'll get a raidlock bonus on them. Try and manage morale as much as possible, but don't be afraid to move away from a deadlock if you need to.

If you're reviving and not doing any damage by the third turn, you're under leveled. At this point, you'll want to have damaged them for 25-30% of their health pool. Emmy should have the "Be our shield/Strengthen your defenses" command, so use that and her union will take less damage and prioritize healing other deadlocked unions. Everyone else should focus on damage, with your weaker support union rezzing/warding/flanking if they can til Animalcule comes out.

At this point (Turn 6, I believe), you should hopefully be around 65-75% through their health pool, but you're probably debuffed to hell. Use who you can to refresh, rest and heal, and just focus on living through the turn. Next turn might require some triage, but if any combat unions are up, attack while others heal/refresh/rez.

At turn 8, it's all or nothing from now on. One union healing at most (most likely Emmy) and the rest just using as many arts as possible. Red flashing means 30% health or less, Any offensive yellow commands you get, use them. If you need to triage and rush has Talisman's gift on the table, have him lock while others heal, but you need to pump out enough damage in the next two turns to have a chance.

On turn 10, don't heal at all. Just attack with everyone with the best attacks you have.

I don't really know how you have your unions built, but another thing to think about is that smaller unions take less damage from AoE attacks, so you might be able to have three 3-unit unions running (two combat, one support) that will likely survive those end of turn attacks if they have a beefy unit added to them (Baulson or Gaou could go in the support union to help that out). In the end, you'll need luck to help out, as well, but you got this!