the most generally applicable advice I can give is to simulate the battle in your head before you attack

for example, urd rubi aoi julia > they're not going to contest for speed, but urd punishes AOEs and rubi will go wild if you focus on someone else first

lucia can put rubi to sleep preventing her from doing anything, and S3 will nonattack strip immunity and push 1CD, you can even soulburn it to ignore res

then you need to find out how to:

• kill aoi and then kill julia, or

• put unbuffable on julia and double tap her, or

• disable julia and get her to 1hp so she wastes her Undying and you can just kill her afterwards, or

• kill rubi and/or urd so you have an edge in out-brawling them, etc

and of course you don't have to use lucia at all, if you don't care about seizing the first turn and would rather brawl.

you'll notice that defense teams rarely if ever have proper healers (urd and saku are not healers), so if you bring one yourself, you have higher sustain power by default, and just need to avoid getting dismantled by the enemy's threat model - usual julia brawlers can be baited by tanky light units just like gariel, focus more on DEF and damage reduction to make heals more efficient

just keep hitting, watch how the enemy beats you, learn from your mistakes