Union events? Indeterminate. They pop up as often as the game developers decide, there's no fixed date. You can find the list of predicted events based on the DMM version here tho
https://docs.google.com/spreadsheets...gid=1090858479
Terms wise... I'll list all I have off the top of my head.
(number)-LB: Number of limit breaks. Number = number of stars you see.
MLB/FLB: Max/ Full limit break. Note that some of the people here like to use FLB for the 4th limit break of SSR weapons, which hasn't been implemented in Nutaku yet.
DATA: Double attack triple attack. Listed as combo+/- in game.
AB: Auto battle (green)
AAB: Auto ability battle (red)
Dispel: Removes enemy buffs
Cleanse: Removes your debuffs
... And I dunno what else you don't know, so someone else might have to fill in the gap for me, or you can continue asking.
Well, this event basically gave us one thing that water doesn't need, but sure as hell made it better: Atalanta. As a free fucking raid hime. I'm sure I don't have to go into what she brings to the table.
cough cough free discount Cthulhu
Ok, but the team I might go for with water... Snow Raphy Triton Atalanta Aphrodite/ Sol/ Kikuri Gabriel(?)/ Saraswati(?). Backup can be Belphegor and Oceanus(?). The (?) himes might be swappable, depending.
I don't think Snow Raphy, Atalanta or Triton needs any explanation. Aphro is basically your water heal, Sol is your C frame atk down should you find yourself needing more damage cut
(and complete cuteness. Yes I'm still not over it crucify me will ya), and Kikuri exists to give you your early burst, if you so orient yourself towards that build. Gabriel I'm not so certain, she completes the A frame part of def break but doesn't really push it to the cap (you get 39% with Gabriel), as is with Oceanus/ Saraswati, given that you basically need to run Ambush with the latter, although you get way better damage.
Soul wise... It depends on how you're running things most likely. I like to play my safest option with Mordred (because VoF), but it might be different for you. As far as EX skills go, you can run BP for the Gabriel in front setup if you're not using Mordred, and you can run Ambush for the Oceanus/ Saras in front setup.
Edit: If you find Raphy's debuffs not working for you (either because they're single target or you need extra 5%), there's always sniper shot EX too. This one works well for me in AQs.