How hard will the Vengeful Demon Challenge quests be?

Double Merlin and Musashi lvl them up and their skills and you’ll be good to go for the last challenge quest

You have merlin. LOL. No challenge quest will be hard for you.

Last year, I "Hercules" through most of them with Waver.

Well I bet you'll be fine. I'll be counting on my Shishou and Jalter again for this one with a couple of command spells on the final round. Cause sadly, i have no merlin nor waiver.

OG Saber will be on rate-up in the Valentine Event with MHX alter.

I do not think it should be that much of a problem. A combination of mashu + merlin will keep your team alive no matter what (granted you know when to use the skills). Basically, mashu + merlin + class advantage servant. If a stage has multiple classes, you could use plug suit and swap your servants to match class advantage. Even the support Dantes you are provided with can do the job tbh. Just keep him alive with mashu and merlin. Besides that, max level your jack so that you dont have a problem with Door 6. Nightingale, though might not do a lot of damage, she is your best berserker and if you put her with mashu and merlin, there is so much survivability for your team.

Compared to Nerofest later this year, you should be able to do the quests. Last quest might be difficult for you given your team. You definitely need to level all the servants you listed before Nerofest hits because there is no way in hell you will be able to beat some of those quests *cough*KH*cough