Can my pet Cinq beat any of the Halloween Challengers?

Every challenger but Nightlady probably, yes

Yes, with the right weapons :)

Which weapons should I equip?

I've equiped... Gold Quill Item Hunter Quill Pumpkin Sickle Blade

You shouldn't have much problems with Deathly Harbinger with your stats. I used Fireside Bow and Zombie Shield for that one.

Lingering Haunt, not sure. I just used random weapons :P

Torchbearer, I used Boots of Deathward and Phaloroceas Rain Charm. It couldn't do any damage to me.

Carrion Lord/Nightlady, there are some set suggestions on the various topics on this forum :)

Thanx ^^