Honestly, if I knew I was gonna do this I should have done it at the beginning of the event. Ah well.
Today’s hardest quest is a bit tricky; first two waves are lancer enemies with the ability to poison, so both Arash and Touta are out of the picture. As for wave three, a Hydra rider boss will fight you, which makes fielding Waver and Merlin more risky. The biggest problem, though, is the free assassin selection; personally speaking, the 1-3* assassins got the short end of the stick in stats and skills, so most of them would only be hitting like a wet noodle on the thing. Don’t worry, though; there are options.
For the best results I would pick either Hassan of the Cursed Arm or Hassan of the Hundred Personas. Cursed Arm isn’t particularly strong, but he’s a very good star generator, and at third ascension gains access to a strong evade option which doesn’t expire until attacked. Against the Hydra, it’s not as useful since its attacks are AOE but it can still save him. As for Hundred Personas, they are exceptionally good for arts looping thanks to their upgrade; gain NP, launch NP, repeat. If done enough times you can succeed. In both these cases, Mash would good as a support, and for waves one through two I would field either Fergus or an AOR berserker like Kiyohime or Erik Bloodaxe.
Another method (albeit more risky) is to go full glass cannon; field Mash, Lu Bu and support Merlin. Use Mash’s defense up constantly and use Merlin’s party invincibility when things are looking ugly for Lu Bu (also use his NP to charge/heal Lu Bu and Mash). At the Hydra, disregard defense and just completely buff Lu Bu’s attack/NP through his skills, Merlin and whatever Mystic Code you have equipped. After that, nuke the damn thing. Due to variables, this might not kill the Hydra outright, but will leave a huge dent in it. And in the event that this party gets wiped, always have a backup. I’d say choose between the Hassans in method 1.
Hope this helps.