This is my strategy for farming the Scathach quest without using any command seals.

Scathach Quest:

Here is my team set up. For the support I've used a lvl 90 Attila, lvl 90 Artoria, lvl 80 Alter etc. It didn't really matter because I usually didn't need their NP.

Wave 1 - Target the Caster first. I made the mistake of leaving it last once and it killed my MHX. During this wave, I usually focus on building MHX and Okita's NP up. I don't always need MHX's NP to kill the demon on the next wave thanks to the Purely Bloom CE so I mainly focus on Okita. Salter is there to use her heal just in case. There are are better healers you can use but I only have her. If the enemies get close to using their NP, I usually one-shot them with a buster from Okita or MHX.

Wave 2 - I focus on the demon with MHX first. I have her at NP2 so if she has her NP at this point it kills him instantly. If not, a few rounds should do it. I usually heal Okita with the default outfit if she has low HP or use her dodge skill.

Wave 3 - All three of them usually make it to the third wave, but usually Salter and MHX get taken out pretty quick. That's fine though. If my support is Alter or Artoria I use their charisma on Okita along with her quick skill and the damage boost from the master outfit. That usually takes a big chunk out of Scathach's health. After that I don't usually need to use anyone's NP again, as my support is usually equipped with Purely Bloom as well as Okita so it's pretty quick after that. Sometimes I'll use an Artoria support with Kaleido and finish it quicker, or Mordred and Okita will fill their NP gauge quickly and finish it.

And that's it!