I finally did it! I got the Thanatophobia trophy and the platinum!
I lucked out big time on this run. I beat Khidr with my first generic knight and also amassed something like 10k gold, which allowed me to level up like crazy. My second guy was kind of useless, but he made some big bucks as well.
My third guy killed Alexander, Ponce, and (somehow) Herodotus! He was a Shinobi with Gigantism, and he hit like a truck. I was one- or two-shotting the wizards that came out of Herodotus, which is quite frankly the only reason I beat him. Unfortunately, I was too huge to be able to avoid the final boss's attacks.
I knew I wouldn't be able to kill the boss quite yet, so I used my next several guys to go gold-hunting so I could level up, and I actually stumbled upon a couple good pieces of gear (Ranger Plate and Guardian Bracers, e.g.). I cranked every dollar into attack power, figuring if I was going to win it would have to be without getting hit anyway, and when I got a Shinobi I went in and beat the boss with her!
This is one of my proudest trophy accomplishments in recent memory.