So I'll give you some similar advice that I gave a fellow union member. If you're playing dark, you should be using a constant damage cut team. It's easily available and could be your main playing style. I'll outline the concept below and you can tailor/modify with what you have and what you want to do.
1. Soul - Either Andro OR Joan work for a constant damage cut dark team.
Andro has a full team heal plus also has her cursed canting chains (~17% damage reduction). I like her with the Shingen spear (+30 BG after burst) but her Planet Star (elemental attack) is perfectly viable. Her other weapon isn't worth it at all. If you don't have a healer, Andro is a solid choice.
Joan has a single 1 turn every 6 turn for 40% damage cut, shortform: (1/6T, 40% DC). I also think the defender weapon (crest flag for an additional 20% damage cut) is pointless. You'll want to take the elemental weapon (saint gauche) for the additional damage. You also have access to Saint's Breath which is (after you get it working) another 60% defense (~37% damage cut).
2. Himes- You have three choices(!!) - Cernunnos (1/5T, 30% DC), Pluto (2/8T, 30% DC), and Nefertum (2/6T, 25% DC). I believe you only need 1 of these and you can go full assault Himes for the other three. All three are actually top tier Hime and can work on this team. With Cernunnos, I'd rather run Andro as Cernunnos has a stacking frame defense ability and so Joan's big EX skill can't be used. If you have Nefertum, you want to use Joan as Nefertum brings healing. Pluto brings additional damage and so is also a solid choice.
3. Kaisers. This can't be stressed enough, but you should buy the Kaisers of each element from the Eido shop and/or save the SR Dragoons of each element from Gacha rolls. You'll need potentially six team configurations for your standard raid. The idea here is to have 1 or 2 Dragoons/Kaisers for on element damage cut for when your other damage cut isn't available. Dragoons are 3/8T, 15% to 20% DC and the Kaisers are 2/8T, 30% DC. Obviously the Kaiser is superior, but the Dragoon also lasts longer and is easier to get. The Dragoon is only 20% at MLB so I'd use Eido orbs to get there. Two Dragoons get you nearly full constant 20% damage cut (i.e. for the dark team that is fighting thunder, I'd put 1 or 2 wind Kaiser/Dragoons into the sub Eid slots.) after 8 turns. This helps tremendously in your staying power.
With Cernunnos and 1 On Element zero star Kaiser you'll have 30% damage cut 9/20 or nearly 50% of the time.
Add in Cernunnos' 29% stacking defense damage cut and you're at 50% damage half the time and 70% damage the other time. Andro's base heals (in this situation) are 70% stronger than what you see on paper. This right here is the bulk of fully viable Dark Team. After this you can build your team with whatever Himes you have/need. You should be able to fight most Raid/Rag content with minimal help. (Right now I and a union mate farm TRags with our Wind teams using similar layouts and can finish the rag without additional help. Neither of us are wind mains, but the DC team works, especially when both bring Andro for group heals.)
Dejnov.
P.S. Bonus Points for Ascension in your grid. For a team like this after assault and vigor, you should only be looking for ascension. The more ascension you have the longer you can fight. It's absolutely essential to find and utilize all sources of ascension to boost Andro's (or other Himes) heals. You already have a 70% buff to her heals and so all ascension you can get is 70% better than advertised. The only problem is that there's only one great source; Nefertem's Staff (assault/ascension). If you get Nefertum, you'll get the staff and the ability to drop the on-element kaisers (if you have Anubis) and can run Shingen instead.
P.P.S. I'd obviously suggest the Guardian Guns as your end game grid (Pride/Strength) for double assault. That will be all the offense you'll need. The chalices (Dender/Resilience) are another source of ascension, but you'll end up doing much less damage than you would with the guns. Go guns!!