If I got Marche's BSB, does that in anyway help his USB or make you better at all?
Well the +10 attack for mastering it. Other than that not really, the BSB seems to focus on making Marche a defensive character while the USB makes him a DPS beast. Marche's defenses are low for a paladin and the current meta focuses much more on speedrunning so if you have both the BSB will almost never get used.
Element wise for me
-Water is probably my strongest, Marilis is the only *4 sub30 and when I'm up against a no weakness boss water is usually my pick. -Wind is my second with Bartz USB and Zack Chain, I can almost sub30 Midgardsormr and I'm hoping the new celerity skill that comes out in two days is the little push that lets me. -Dark is probably next with Raines BSB + Kuja and Riku USB, no chain through. -Holy is also really strong thanks to Marche, but he's pretty much carrying the team. I have Orlandeau's OSB and Agrias USB and two r4 assault sabers but I don't think that's going to be enough for Hades. -Fire is really strong as well, Ace USB, Papalymo BSB&LM, Terra USB+LM, but I can't sub30 Isgebind yet, I need a chain or more level 99 Marilis and King Bombs. -Ice I have Rinoa's chain and Lulu's USB but not much other than that. I can sub30 Fenrir easy but I don't think I have a chance at Tiamat unless I hit the Squall jackpot tonight. -Earth I got Dorgann's USB and Bartz, and Marche through it would really help if I had his LM Relic. I'm also hoarding up earth/holy orbs from farming torment Belials in preparation when Earth Saber comes out and Dorgann becomes a earth beast, but since that's a month or two away I still consider myself weak in this element. -Lighting is my weakest for sure, I've got Lightning's OSB and supers, Orlandeau's OSB and Arnea's OSB and BSB, other than that nothing besides magicite unviable BSBs and Ashe OSB. Bismarck is the *3 magicite I still need luck to sub30. Really hoping for Lightning USB/AOSB tonight. -lolbio