For our last event before the transition to lv90++ farming nodes (we start this summer with Best Clay), we are blessed with a 30/50% NP charge event CE.
The starting charge, combined with a single Berserker mob in wave 2, leaves room for many different farming comps that don’t require Plugsuit nor double supports (you can farm bond with any generic DPS with 30% charge if you have either Ozy or Taigong): heck, we also have a F2P comp with old welfares!
Of course, if you are lazy Buster looping works too, if you missed the usual double Castoria comp.
EDIT: Habetrot doesn't need her append skill, just use both Castoria S2 in room 1 and Habetrot will refund the extra 20% needed (my advice is to lore her append regardless, if not now in the near future: having one unit that can charge 100% with any CE and then die is too good to pass).
PS: Depending on your welfares availabilty there are other F2P comps I can think of: for example using more recent units the team Habetrot, Shuten-Douji (Caster), Elisabeth Rider (lv1 append), Oberon works with base event CEs and doesn't require plugsuit (any 10% charge MC works if you don't want to lore Elisabeth append, I have lol, she is too good): Shuten has special damage to Demonic in room 2, while Oberon does his magic in room 3.
If Shuten Caster is too old, you can use Da Vinci Lily (Ruler) in her place: she kills too in room 2 with any MC buff (even without considering the guaranteed crits in room 2) & you don't need any append skill.