If Marian is used as a DPS:
Marian's low per-shot damage means she doesn't really gain much damage from per-shot bonuses like those provided by Phantasia or Little Sunshine.
She does deal some skill damage, but the vast majority of her damage is ballistic, meaning skill damage buffs aren't overly useful for her.
Instead, she mostly wants generic ballistic damage buffs, as well as supports who can provide generic or chaos damage resistance debuffs.
Yao - Nightglow
Yao's ATK and ballistic damage buffs are very useful for QoP.
Marian's high crit damage value means that Yao's crit damage mods are very strong for Marian.
Vidya - Celestial Swan (and other resistance shredders)
Celestial Swan and other resistance shredders (Acacia - Kaguya, etc.) that reduce the enemies resistance help enemies take more damage.
If using Celestial Swan, make sure the other support isn't off element to Enigma.
Fenny - Starshine
Fennys ballistic damage buffs are useful for Marian, and not needing to reload allows her to stay in ADS for longer, increasing her buffs and damage.
Eatchel - The Cub
A Blitzing Fangs Eatchel provides healing as well as a decent ATK buff.
Not the strongest support, but still good.
Mauxir - Shadow Ka
More damage, and a target that doesn't move.
Shadow Ka is also on-element to Enigma, so you can use Enigma + Shadow Ka + Celestial Swan if you wish.
Fenny - Coronet
Cornet's RoF buff is a nice buff for Marian.
Since there really aren't any 4* supports that actually help Marian to any significant degree, and basically all 5* operatives will out-damage her, you're generally best off not using Marian as a DPS.
If Marian is used as a support:
Marian provides a strong chaos damage resistance debuff, allowing her to support any operative that deals chaos damage without exception.
If used with Deep Sea's Call and Amarna, she can work as a very good support for Fritia - Turbo, who likes the ATK and ballistic buffs, and the chaos damage resistance debuffing.
If used with Twilight squad, QoP can be used as a strong support for Cherno - Enigma, as Cherno's damage being concentrated into her standard skill detonation means that even short duration buffs can be a huge dps increase to Cherno.
To a similar but lesser degree, QoP can also be used to support Nerida - Styx Envoy with Marian - Riptide, who also deal a lot of chaos damage over a short period of time.
QoP can also be used as a support for Acacia - [Redacted], who deals entirely chaos damage, increasing Acacia's damage by a large margin.