I'm sure it's no big deal to you guys, but after about 4 or 5 attempts to beat Legendary Seaway that consisted of me trying so hard to combo high enough to break enemy defense on the first floor, failing, and then getting one shot, wasting 88 stamina near instantly, I finally beat that shit. Went in with GZL lead and my best green pronger subs and Sylvie for the HP boost so I could tank a couple hits (and emergency oh shit heals, or heal break with CuChu for burst) and it worked perfectly. My mistake was trying to use Bacetet the first handful of times when I know god damned well I can't combo to save my life. Feels good to have that out of the way but now I get to try and formulate a plan to do it in challenge mode...
Congrats! One step at a time, just keep plugging away at content and you'll be clearing challenge 10 in no time.
As for challenge mode on seaway, I stopped bothering at a certain point. Haven't really thought about it in a good year. Maybe I should go back and try now that we have some insane leaders.