I managed to beat the Challenge Quest too about a week ago! I used a team of Jalter and two Merlins (one was my own with the Prismas Cosmos CE, the other was a friend's Merlin with the 2030 CE), and I managed to not lose a single servant! The most nail-biting part for me was the second round with Fergus and the Chimera because Fergus kept going after my Jalter (and she ALMOST died to him too) along with the Chimera being a pesky Berserker.
Other than part, I didn't have much trouble. By the last three rounds, I kept spamming Merlin's NPs over and over again to the point that I had a good supply of HP and NP regeneration every turn to keep both Merlins and my Jalter alive, and Jalter even performed some quick shocking kills thanks to the added Critical Stars from the 2030 CE on my friend's Merlin.
Edmond Dantes didn't give me too much trouble, mainly because I played things safe by using Merlin's Invincibility skills on turns where I predicted Dantes would use his NP.
Yeah, double Merlin is just too good!
The hardest one for me was actually the Amakusa + Jeanne tag-team. I expected Fergus and Caligula to be the hardest parts, due to Fergus with his NP Charge plus the Chimera and Caligula with his Criticals. Thankfully, each time I felt he would attack, I managed to stun Caligula each time with Waver's NP and I managed to take down both the Chimera and Fergus down quickly before they did some real damage.
As for the 2 Rulers, Amakusa wasn't hard, as I defeated him quickly. It was Jeanne who kept giving me trouble due to the fact that she would go after Vlad III the most with her attacks and it was hard to keep him healed. Dantes was just tense because:
1: I knew he could potentially wipe out one member, especially Vlad, if he focused on them.
2: My skills wouldn't keep his NP down for long and I didn't have a skill to give invincibility and even then, he had the potential to use his Invinci-pierce followed by Golden Rule followed by his NP.
3: The most tense part was having to make the risky choice of putting Evade on Vlad III in case I didn't stun him with Waver's NP and risked him using his Invinci-pierce. I didn't stun him, luckily he didn't use his skill beforehand, but he almost took out Tamamo with a Crit and would've taken out Vlad III had I not put evade on him.
Either way, thankfully, Vlad III was able to use his NP enough times to get the lucky Crits and defeat him.