Diana/Lucus/Nia 9 bullet+cupidshot/turn

This deck guarantees 9 quietus bullet cards + cupid shot per turn. Use Picky Palate to exhaust Pistol. Use Diana's ego skill when you can. Nia's gearbox will almost give you enough ego to use it. You need to be careful not to use WAMH before creating space for your bullet cards. You only have 2 non-upgrade WAMH so you only need 1 Adago to discard them both. If you have 2+ Adago you can get rid of one by using it and not discarding anything.

You can get a bit more damage by getting weakness attack on S.S.S and the attack WAMH. Cupid Shot isn't really weak to elasticity due to it hitting only 10 times. The damage loss is minor. The Blooming Love card that discards all bullets isn't a good alternative to Cupid Shot because it's a skill card and does not benefit from many atk buffs such as the +30 bullet persona, +150 atk personal or +50 atk engravings. It might be more useful for multi-stage content but I haven't tested how it works there.

Pros

This deck is incredibly consistent. After using your upgrade and exhaust cards, there are 5 draw neutral cards and 5 non-draw neutral cards. You will always end up with your 5 non-draw neutral cards in your hand.

It deals about 150kish per turn depending on gear.

It has 10 cards after using your upgrade and exhaust cards, but one of them has haste and 2 of them will be discarded so you'll only use 7 actions. If the boss isn't immune to action count increases, you'll use 5 actions before equipment effects.

It has extremely high healing due to Diana fully utilizing Nia's Soul Rift

Cons

It uses a hand size of 10. If this bothers you can get rid of one of Diana's WAMH.

It's not ideal for multi-stage content, but can still easily clear them. The problem with multi-stage content is that if you create 9 bullet cards and only 4 is needed to kill the enemy, 5 bullet cards will be in your deck in the next fight. If you kill something with Cupid Shot, you'll have 9 bullet cards in your deck.