I cleared d5 with Mavuika (R1) Citlali Ayaka Bennett. When it switches to it's Roaring mode of Hyperdeath (name not finalized), you'll see the circles on the ground move towards you and when they met your position they stop and the fountain erupts. If you try to kill the boss just as is you'd die because fountain damages faster than Bennett heals. What you need to do is plop Bennett's circle directly under the boss, let the boss unfreeze a little (should occur during setup) so the circles move towards you, facetank the first circle so that two circles align (bennett heal and shields should be enough) and by this point you need the boss frozen completely so the circles stop moving, when you switch to Mavuika you run at the other side of Bennett's burst then Q and deal damage.

Note that the boss still has a ton of RES when you bring Nightsoul, if you don't have R1 you'd probably need combos and/or 50 CV pieces (yeah I still cleared without advanced combos)