Yeah ingame timer would be the only real way to do it. I'm working on a sub 4 second run of Freud right now - but it's down to FRAMES and I can't figure out how to get it done faster.

I tried to upload a comment/ video of the run - but it got rejected for some reason. My rules would be that the in game timer must be visable - even if a split timer is used - though with hundreds of runs of 5 seconds or less on them - I have no idea how the timer's gonna keep up, and no mods.

For some reason I can't upload the video.

Speedrunning Arena fights sounds like a lot of fun.