I improved this! Check out my 40:07 run here: https://www.youtube.com/watch?v=FLAZYuBIvvQ
100% single-segment speedrun of Demon's Crest, SNES NTSC-J version, done on BizHawk 1.9.3 Compatibility Core, non-TAS. This run gets the "good" ending by collecting all HP upgrades and items, and beating the true form of Phalanx. Timing starts when I press START at the title screen and ends when Phalanx 3 receives the final hit.
Quite a solid run. It is possible to improve if considerably, but considering all the RNG in this game (bosses in particular), it would really be difficult. Arma 2 wasn't the best, and getting a super quick kill on Phalanx 3 could save another 30s, but that is so ridiculously inconsistent that I'm not planning to get it in a run. Here, most bosses were really good, and execution was decent overall.
I'm certain a sub-40 is possible, but it would require absolutely sick RNG and execution.
Enjoy the run!
Watch the Twitch live version here:
http://www.twitch.tv/le_hulk/c/6737093