I'll try to avoid spoiling too many details as I think it'd take fun out of planning and thinking through the builds.
I'd divide them as follows:
Free for all
Theoretical lowest possible level: 11
Likely achievable lowest level: 11
Current holder: Dragonslayer (completed at level 11.625)
Under this category fall all kinds of accounts; no holds barred. Everything can be aided by other accounts, which makes it possible to aim for the truly lowest possible level. What defines the lowest possible level is the fact that Elvarg's breath can and does hit 12s, so in order to stay alive when maging her, one needs to have at least 13 hits to begin with. Other than that one needs of course 31 magic for one of the map pieces. These limitations bring the combat level to a bare minimum of 11.5, and one can even gain three magic levels during the fight in order to stay at combat level 11.
Ironpeople
Theoretical lowest possible level: 12
Likely achievable lowest level: 12 or 13
Current holder: Dragonborn (completed at level 13.625)
This category contains all accounts with a trading restriction. As the code doesn't currently enable ironpeople to complete Shield of Arrav, one needs to complete Restless ghost instead in order to gain 32 quest points to be able to start Dragon slayer. The prayer xp reward from Restless ghost already pushes the combat level to a minimum of 12.125. Based on my own experience I can tell it would require some good luck to be able to complete Dragon slayer without reaching combat level 13 as an ironperson. Still, it's possible.
I'd advise doing the run without using alts or friends to help in any of the quests, as it's very rewarding to solo through them!
The Ultimate Challenge
Theoretical lowest possible level: 12
Likely achievable lowest level: 12 or 13
Current holder: Dragonborn (completed at level 13.625)
This category is almost the same as the previous one, with the one big difference being that it contains only accounts with both trading and banking restrictions. No banking requires more planning, but also creates some high stakes situations, as one cannot dump valuable items to a bank, or any items at all to be precise! Surviving three hits in a dance with an ice giant with 2 hitpoints left when trying to mine blurite really gets one's blood pumping!
As was the case with the ironpeople category, I'd advise doing everything without the help of an alt or a friend, as it makes the run more exciting and rewarding.
Melee only
Theoretical lowest possible level: 35-40?
Likely achievable lowest level: 35-40?
Current holder: Logg (completed at level 53)
"The way it's meant to be done". No maging the Lesser demon in Melzar's maze, and especially no maging Elvarg from a safespot behind a broken pillar! "I like to bash things close up", as some would say. I don't know if Logg actually beat Elvarg with melee, but based on the combat level I'd like to think so. I haven't done any theorising on required combat stats to be able to beat Elvarg, but I'd think one would need at least 40 attack and a rune weapon, and at least 30 defense and addy full. Maybe, just maybe under 40 attack and an addy battle axe might do the trick too?
Club 22
Wield a rune plate mail body/top at combat level 22
Current members: None
This is a special one. I've calculated it's possible to wield a rune plate mail body or top at a combat level of just 22. I'll let y'all figure out how, but I can say that to be a member of this club is unfortunately incompatible with the lowest level builds.
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Having finished the UIM build on Dragonborn, I think I'll go for at least the level 11 run too. It would be fun to try the melee one also. Maybe even the Club 22 build, although that's more of a boasting one
Edit on 22.10.25: updated the hiscore for free for all category having finished my level 11 run on account called Dragonslayer a couple days ago.
That was a smooth one compared to the UIM build! I had to get 32 magic though, as the code doesn't currently have the correct boost for Wizard's Mind Bomb (only boosts by 1 level instead of 2). I also realised you can actually go as low as 11.25 combat level, so perhaps I'll do one low level run more once Wizard's Mind Bomb is fixed.