Limited Zero/Projection for the buster performance up for more damage. Volumen Hydrargyrum for suitability if you feel as if you need it more. As far as support servants go you would want Merlin, Waver. Banana is very vulnerable due to not having a guts or an evade/invuln so the 2nd skill from both waver and merlin will give her a bit more staying power. As far as CEs go for Waver/Merlin you honestly can never go wrong with double 2030 or Prisma Cosmos.
Supports for Ibaraki-Douji
For long battle or battle of durability, her short CD Def Up Skill + steel training or Holy Shroud of Magdalene will make her almost unkillable. With a Waver or Mashu, you can tank your enemies NP like it's nothing.
If your enemies like to spam def ignoring skills, use Guts CE instead.
Zerker that can deal high damage + can receive the blow back, Ibaraki is truly an awesome unit.
I often called her a female Juggernaut.
General teams or Buster teams, use Buster CEs.
Her buff is Atk UP + NP Up and def down after NP, Buster Up will make a complete multiplicatively damage.
High base atk + Zerker 1.1 atk modifier + effective to almost all job, 150 % damage + Buster UP * Atk * NP Up and It's fucking very powerful, if you can manage to use her NP again then it's pretty much the death of your enemies ( NP UP + Atk Up + Def down + Buster UP = GGWP massive damage ).
She is also useful when you're dealing with annoying enemies with lots of buffs. She has buff removal on her NP and this can turn the tide of battle.
Bonus : i have her level 80 NP 2, she is second only to Heracles.
Any starting NP ce does wonders for her, cuz she has a bit of a hard time generating NP. The best would first sunrise so she can have the trifecta of buffs (buster, attack and NP up). For supports obviously Merlin and Waver are good choices, but on a budget Shakespeare works well with her, as well as Leonidas for taunting to keep her alive and giving her a buster buff.
I want to recommend, besides the obvious Merlin and Waver, Blavatsky and Shakespeare as a bit more of a budget supports. The latter being more for bursty situations and/or disposable buffers with plugsuit, and if you have his strengthening, some NP Battery. Kind of the same for Blavatsky but she'd fit better for a bit of wave clear here and there with her NP, and also a bit more sustained damage with her 3 card types up, of course her NP Battery with Mana Tuning and some crit stars here and there. Georgios and Leonidas are awesome options too, when it comes to tank damage. Specially Leonidas since he has a team wide buster up and provides better stars with less gather than Georgios.
On CEs, well, the menitoned Limited/Zero over, Volumen Hydrargyrum, also Golden Sumo which is pretty much a standard nowadays if you have it, Art of Death is another option if you face humanoid enemies, Kaleid Ruby From Prima Codes event which also have the bonus of being 4 Stars free MLB so less cost but still viable.
Others worth mentioning are Partake with the King and First Sunrise, being 10% Buster Up and 50% NP and 15% Buster up and 40% NP respectively and if you have Joint Recital, you can make a buster crit comp with it.
I got Ibaraki from the same way and i put Kaleid Ruby gets you a increases Buster performance by 15% and a increases NP damage by 10% (MLB) beside have a cost of 9 so you you have more options for CE on your team.
Her role in the team its generate the more damage posible.
Maximize her third for a bust on def and put some tank with her, for example Mash or Georgios
Ever since I got Partake With The King, my Ibaraki has pretty much always been equipped with it. The starting 50% NP gauge and the increased Buster damage are useful, since I try to rush the first few waves as fast as possible to prevent Ibaraki from suffering too much damage before she can get to the final boss and unleash her NP. At that time, my other characters will have charged up their own NPs as well, which they can launch right after Ibaraki's NP so that they can benefit from her Defense Down debuff.
Berserkers are super vulnerable, and I've had unlucky experiences where Ibaraki was brought close to death's doors on the first turn even with her max level defense buff activated, and Ibaraki should be one of the particularly bulky Berserker servants to have higher survival power because of that skill.
Really, I just recommend anything that can help Ibaraki survive longer and unleash her NP by the last wave, or failing that, have other Servants who can quickly clear the waves and mitigate the damage on the team as much as possible.