I keep dying on the Lunatic version of the BHB... I gotta look up strategies at some point. I'm enjoying Arena Assault, though! The battles for your entire streak are based off of who you send into the first battle, so if you ever struggle in it just send weaker/less inherited characters into the first battle and the rest of your battles will be at about that level!
I've just been sending in a 4*40ish Askr trio + 4*40 Olivia into my first battle, which makes the rest of the chain on Advanced relatively easy while still maintaining a respectable score. Another thing people have been doing is sending in a solo unit for the first battle, and if you get past that - the nice part is there's no consequence to failing your first battle since Arena Assault doesn't cost swords or stamina - then the rest of your matches on Beginner will be solo fights no matter how many units you bring. Not so good for points, but it's a super easy way to get a lot of items fast to help you with your real matches!
I'm really loving this new update in general, especially the ability to refresh Training Tower! It makes grinding people up a breeze since you can just keep refreshing until you get an easy matchup for them. Yesterday I finished leveling my 5* Boey and Leon, got a Selena up to 4*40, and got a Sully up to 4*40 from 3* in the span of a couple of hours. Pretty rad considering the EXP gains and difficult matchups used to make it so it'd take a couple of hours to grind up one person! I'm just kinda going through my list of ideal-natured units and getting them up to 4*40 now since it's so quick and easy to do now!