I believe this is the first 5BC speedrun in the world. Hope you enjoy the run! Comments and timestamps below.

After a few dozen attempts which mostly ended in Stilt Village or Castle, I have finally managed to finish a speedrun on 5BC! I went with a Giant Killer Survival buil with Denial Wave as an off-color skill. This is definitely not the best build for speed, but it allowed me to actually survive through the insane armies of enemies in certain biomes. I originally tried Tactics, which was faster and worked very well until Time Keeper, but Castle felt impossible as hordes of Demons, Bombers and Arbiters are guaranteed to hit you a few times, immediately sending you to 1HP on a purple build.

The run relied on a few important strategies:

* Go fast: well that's our good old Assault Shield

* Get at least 25 Survival stats by HotK: to do that, especially considering I was losing 2 potential stats due to denial Wave, I needed to hunt for cursed chests and to explore enough to find all scrolls in most biomes. For this reason, I took Acceptance. It's also the reason I went to Sepulcher instead of the faster Clock Tower, as it holds the most stats out of any biome.

* Get decent gear somewhere along the run: the starter items, in particular my main weapon Giant Killer, are just too weak to be used for the bosses after Concierge. Because of this, i had to look for some loot in chests from time to time.

* Low cooldown panic button: this is why I used Denial Wave even though it's not a green item. I absolutely needed a way to get rid of huge packs of enemies, in particular Bombers which followed me everywhere. Without this item, taking cursed chests was not realistically possible.

* Room cleaner: this is where Powerful grenade shines. while Denial Wave pushes everyone away and prevents damage, the grenade was used to clear packs of enemies at once, especially during curses. On a lower difficulty, I would have taken 2 grenades and tanked hits during their cooldown, but with the threat of the Malaise it was not reasonable to have more than 2 seconds without any skill available.

* A way to deal with Elites and bosses: that's where Giant Killer comes in handy. Clearly melee is not the quickest way to kill bosses when getting hit is so dangerous, but Giant Killer has such a strong critical damage on the third and fourth hits that it's actually reasonably fast on its own. The other reason this weapon was so useful is that it kills Elites very fast and stuns them, which neutralizes some of the most problematic encounters in the run.

Apart from that, the main strategy was to not die and keep my flasks as long as possible, hence the Necromancy and Dead inside mutations.

Timestamps:

Promenade 2:02
Ramparts 4:26
Concierge 8:48
Stilt Village 10:32
Sepulcher 13:50
Time Keeper 18:16
Castle 20:04
HotK 24:50
Astrolab 27:40
Observatory 33:10

The beginning went quite well, I wasted some time in Ramparts but it allowed me to get decent stats for Concierge. Then it went relatively OK until Time Keeper. As usual, Castle was a big issue and I got lost quite a bit, although I'm proud of killing Elite Slasher without a flask (it's very rare for me on melee build). HotK fight was pretty good. Astrolab took me quite some time, first because I don't know this map as well as the others but also because an extremely weird encounter with an army of Failed Experiments (see at 28:15) threw me off and I couldn't concentrate for a while haha
Collector fight went relatively fine, although it was quite long. There's probably a good timesaver to find there.

Overall I'm proud of this run!