Thanks.

Should I try to max Yag even if I have to do it on Expert? Or wait further beyond that? (Since event after is raid, I'd be able to stock up a bit then, possibly.)

Thinking about it more, Ultimate might be unrealistic. Water might be weaker than wind. Does that change the assessment?

For this event I'm running
Cassiopeia (EX maiden's prayer)
Cybele
Zephyrus
Nyarlathotep
Aurora
Lilim Luxuria (+65%)
+13.5% from weapons

Yag team probably would be
Cassiopeia (EX Sniper Shot)
Nike
Nodens
?
?
Behemoth (+30%)
+13% from weapons

For 4th/5th characters, since I don't have any water debuffs or buffs, my choices are R water attackers, or go off-colour (with Nyalarthotep, Aurora, Zephyrus, Metatron being the likely top candidates).