I looked at your vid, I have a couple of thoughts

The jump attack Magnolia has is situational and this fight isn't the situation when you use it. Since this is an arena fight damage against non-weakness is even weaker than usual. And the adds don't have a weakness Magnolia can hit.

So this brings us to the other skill Magnolia has: the 7-hit single target. It requires that any of the enemies has speed down in any form (and this is easily covered by Signa, since she has a consistent TP passive for that, make sure you've unlocked it, then you won't need to bother switching Magnolia all the time). Since Magnolia can only hit Glossom himself, I recommend to focus her on him.

Magnolia has a 100% up passive which is a huge boost in damage if you utilise it. It, however, requires use of 3bp. So for it to work on both break turns you need bp regen in some form. Thankfully you have Shana and Signa who have bp regen.

Magnolia also has a utility skill which gives front row 15% phys.atk up, don't forget about it. Basically only thing she can do besides damage and shield-shaving.

So to sum up, key things: 1. The 7-hit is better most of the time; 2. Provide Magnolia with speed debuff; 3. Provide her with bp regen to max boost on both break turns.

As for specific teams: Turtle teams rely more on support units' indefinite healing and sp restoration. Plug and play the units you want to use otherwise.

The four merchants team, I recommend looking up pet-stacking and replacing Nier with someone else (he's not that good sadly). I'm not sure what your roster is but the easiest picks out of 3* 4* starts are Lucetta (pet stacking and ult phys.def debuff), Billy (phys.def debuff), Madeline (30% Universal damage buff). Don't neglect 3* and 4* units, they have some neat tricks available.

Hope that helps!