The Golden Army needs to be commanded by someone with royal blood -- I got that. It can also be contested by another 'worthy'. Sure thing. So, elf-prince gets the crown, and commands the army, and Hellboy contests it. Fight ensues, yadda-de-yadda, stuff happens, someone dies, hearts break.
Now here's my thought: Hellboy was not needed at all.
See, elf-prince gets the crown. Elf-princess could contest. If she contests, the Golden Army's stumped until they duke it out. The elf-prince won't duke it out, since any harm done to her is visited on him, and if she dies, he dies. So... stalemate. Nobody gets to command the army. The elf-prince's plot is foiled, the elf-princess lives, and gets to hang out with her boyfriend-to-be.
As a gamer, I'd see this as a gaping hole in an adventure. Sure, Hellboy gets to be all cool and beat stuff up, but... really? As a player I'd be standing back, looking at this, and going, hey, princess, you can stop this!