These items cure Scarlet Rot, which is a poison that Malenia can inflict on you in Phase 2 of her fight. Keep plenty of Preserving Boluses handy.It does awesome damage, can help stagger and bleed Malenia, and will even infinitely drop Warming Stone items that slowly heal both you and it in an area-of-effect if you have them equipped. The Mimic Tear creates a copy of your character that is much tankier when fully upgraded, complete with your armor, weapons, spells, and equipped items. The Mimic Tear Spirit Ash Summon is extremely useful in this fight.Bleeding her removes a huge chunk of her health, which helps counteract her healing quite a bit. I beat her using fully upgraded Power Stanced Blood Uchigatanas, but other Bleed weapons work well too.
Malenia heals herself when she hits you, so dodging her attacks is crucial.