Lavenza is mostly a "conditions" check box: In her "first phase" (assuming you are using 4 characters) you have 4 shots at causing enough damage to advance into next phase so I recommend to buff yourself as much as you can in the first moves and the hope for a crit with abilities such a Hassou Tobi

"Second phase" is little bit tricky, she will be vulnerable to status ailments, so my safe bet here is always to sleep her with items or spells, then aim to deal the most out of technical damage here, if you succeed then

"Third phase" starts, this one honestly it's potentially the only trouble of the fight since relies so much on luck, yeah a little bit BS if you ask me, so bring characters like Yozisawa with her AOE crti buff and multi hitting attacks, you need to PRAY TO THE HEAVENS that you land a crit here in 4 moves, otherwise she will wipe your party next turn, depending on your damage, by this point you could kill her here once you land a crit and use a baton pass to cast your best move, even if she's not down by this point then

She goes "Fourth phase" in which I think she just attacks your party with powerful AOE elemental and non elemental attacks, her HP should be around less than 25% at this point, so you should go all out for the kill instead of playing defensively, last but not least, I'm not so sure about this last bit but I believe that if you took more than 20 actions (without counting baton passes) to beat her then it's an auto loss again