A servant I like to use a lot in Grailfronts, or whenever I have an opportunity really, is Galatea. But she works great in the GF format, she's tanky as fuck, has stacking guts, hits hard from Buster and also refunds well off her NP. And her heal + cleanse is one of the most satisfying skill trigger effects in the whole game for me. With her you can also just field a damaged unit on her side and next time they're together you'll have a heal/cleanse bomb waiting for both when the skill is active (which is often during Grailfronts when the buffs stick while moving outside or with other units), so she can act as a strategic moving medic too (all the better on double heal spots).
I'm actually hoping there will be some harsh traps or stage effects that willl change status on our servants in this Grailfront version, so that her cleanse can get some valuable use.
Another pleasant combo with her I found is having someone who covers NP charge per turn, like Barghest and now Wandjina, so they're complementing each other with heals and charges per turn when side by side. She's so fun to use there.