You can get someone else to kill her for you, or you can just keep grinding normally and you'll eventually outgear her and one tap her. Any season objective you complete even before it's unlocked will be there once you progress it. It is a retarded fight, I had to redo it a bunch of times cause I wasn't outgearing her enough.
Mephisto is also pretty retarded but at least you have a chance to dodge his bullshit even though the tracking and hitboxes on projectiles is completely fucked up but you can see them. Lillith hits you with an unavoidable nuke every few seconds of the fight that ramps up(some enrage mech, but terribly designed) and the ground pops spawn way too fucking fast to reactively avoid so you basically already have to be moving to dodge them.